The development of a new game is an event that, in itself, has little of necessity and inevitability; nothing, in history, necessitated the appeareance of chess, nor anyhting decreed what the rules or the pieces of such a game should be.
Inquiring into the origin of a game often means exploring and analyzing all those specific conditions that converged to determine the birth of a game in the particular form we know it; it means appreciating how sometimes unique events came to happen thanks to a chain of fortuitous coincidences and circumstances.
This holds, in my opinion, for RPGs and their development. Here, I do not want to review how RPGs came to be: excellent historical reconstructions are offered, for instance, by Peterson  and Appelcline . Instead I would like to remark upon some aspects of this phenomenon: (i) its contingency, (ii) its fragility, (iii) its chorality, and (iv) its innovativity.
Contingency. I imagine the birth of RPGs as absolutely contingent. While storytelling is an innate feature of humanity, the specific form of storytelling in an RPG (choral, interactive, supported by rules, enriched by actual randomness) seems to me a possibility which need not have arisen. We could imagine that RPGs may have been developed ten years after they actually were born, a century later, or maybe never.
Certainly, the historical context of the late 1970s provided many desirable conditions for such a birth: from burgeoning wargaming communities to the success of paperback fantasy and sci-fi novels, from the availability of spare time to disposable incomes . Yet all these conditions can be seen as (somewhat) necessary but not (at all) sufficient. The cultural scene of the US in the late was a fertile ground, but not in all fertile ground do trees sprout. A history without RPGs seems absolutely conceivable.
Fragility. It is not the simple confluence of external and social factors such as the ones listed above that can explain the appearance of something as unprecedented as a RPG. If you reduce the scale of wargames to single individuals, you are still far from RPGs (you may get Chainmail); if you use fantasy miniatures with a background, you are still far from RPGs (you may get The Battle of Helm's Deep); if you ask for some degree of impersonation, you are still far from RPGs (you may get Diplomacy); if you introduce a referee, you are still far from RPGs (you may get Strategos) .
RPGs are not born out of incremental steps as the one listed above. Nor are they born by the blunt juxtaposition of many of them. For RPGs to be born requires a patient and passionate melding and tuning of ideas and rules. This process has an intrinsic frailty, in that nothing but personal interest and motivation drives it. Nowadays the landscape of RPGs is rich with games of varied form, content and style; the first RPG may have probably landed on (almost) any of those solutions, or something still unconceived.
But, most likely, the process could have just broken down. In the end, the idea of joining storytelling with some rules is not so unprecedented. Greek theater had regulation on the plays you could present at a festival; Renaissance plays were based on canovacci and pre-defined characters; Japanese courtly poetry required exchange of verses in pre-established forms. The idea of bringing together free storytelling with rules and frames to direct it may have occurred to many individuals, but it did not produce lasting forms of entertainment or expression. The likelihood that the imagination and the efforts in the 1970s turned out to be a passing activity must have been overwhelmingly high. A few accidents may have turned the whole enterprise into a family story.
Chorality. Shaping RPGs in their primeval form required a lenghty process of experimentation which meant bending rules together, running trial-and-error game session, integrating feedback, tuning systems, and more importantly having fun with other friends.
The success of it all, despite being spearheaded by a number of individuals who are deservedly remebered by name, is also a success of communities - whose identity is painstakingly preserved by some . Communities were instrumental, at least, through their positive and receptive response. Like in our now-everyday RPG experience, we know that the success of a campaign is rarely the result of the work of a single individual: the DM has normally the greatest merit, but the contribution of each player is also essential. In spite of the dedication of a DM, a session may fail because of the half-hearted (if not undermining) engagement of some players.
RPGs are social games, and so their precursors could not have flourished without a community providing enthusiasm. RPGs could not be born in the abstract. Like our modern-day sessions, the earliest RPGs required an engaged and open-minded community. In spite of the dedication of the original developers, their projects may have failed because of the half-hearted (if not undermining) engagement of other wargamers and boardgamers.
Innovativity. It is also not immediate to grasp how innovative the introduction of RPGs was. Growing up with RPGs induces one to consider this activity almost a natural form of entertainment. This is a known bias: once a solution is presented, it often seems trivial, but getting to the solution first may have required large amounts of research; gravity as explained in school may seem self-evident, but understanding and formalizing it was a huge intellectual achievement. Great ideas are often obvious once expressed.
For someone living in the 1950s, the idea of something like RPGs may have been completely alien. It was not probably as remote as, say, modern electronic gadgets, but equally unconceivable because it was definitely outside of everyday experience. This is in part reflected nowadays by the surprise and questions of some first-time RPG players, puzzled by the aim or the dynamics of the game, especially if they have never played any RPG-derived product (like CRPGs).
The radical novelty of the RPG is witnessed by the challenge of defining it. Once published it was evident it belonged to a new genre of game, but how to categorize and name it? The search for a definition, and the slow process by the community to distill an agreed upon name demonstrate that they were dealing with something not reducible to any preceeding category .
Looking back to the development of RPG in the crucible of the American 1970s, this process may appear simple and obvious. It is not easy to fathom that the success and the form it acquired is the result of the particular efforts of many individuals, friends and groups.
That something so original and, within the field of entertaining and storytelling, revolutionary could have an origin so contingent and fragile is, I think, vertiginous and beatiful.
 Peterson, Jon. Playing at the world: A history of simulating wars, people and fantastic adventures, from chess to role-playing games. San Diego, CA: Unreason Press, 2012.
 Appelcline, Shannon. Designers & Dragons. Evil Hat Productions, 2014.