People tend to adapt to new ideas in three different ways. Some just jump in and go, even if the new idea isn't fully formed. Those who jumped in and started DMing 5e when it was still in the play testing stage. Then you have the largest group, those who are more cautious. Those who play in those games but until they feel comfortable will run the games they know, this process can take years for some people. And the traditionalists who have to be drug kicking and screaming to the new idea. Which in gaming there is really no force to drag them beyond the peer pressure of friends, not something you usually see in PbP.
I ran 2e games for the entirety of the 3.0 era, only switching once 3.5, the improved rules came out. Two years is still a pretty short amount of time for people who have refused to switch to the last version to accept the new one, whether it is better or not. Given the number of running games it would seem 5e is very nearly as popular. Given a couple more years it is hard to say what will happen.
Polkabear the SRD is fine to create characters with and play, but most people are not going to run games without having full versions. The number of players doesn't seem to be an issue. It is a matter of what system people choose to run their games in. Cost and time are certainly barriers to that. It usually doesn't take as much time to be comfortable to play in a game as it does to run it. The SRD helps get players involved and some of those will choose to adopt, but it is a process that takes time.