Thanks everyone, that's quite helpful.
It looks like another category that has come up in the discussion is:
4) They aren't ready (haven't finished a current campaign, waiting for the system to mature)
Also:
5) The've moved on from D&D
I didn't really focus on 5, because that's a whole different thing. I personally love trying and playing a variety of non-D&D games. I'm mainly thinking from the perspective of "what do you play when you play D&D?" I'm counting Pathfinder and retroclones in that mix, but not much else. (I don't know much about 13th Age, but the impression that I get is that it doesn't easily fit into the D&D family of games.)
Number 4 is an interesting one, because it's something that should change in the next couple of years. In that time, most of those who aren't looking into 5e will have finished up with what they have going on. They may or may not go for 5e, but at least 4 will mostly be off the table.
I'd say the love of the greater granularity/crunchiness/tactical possibilities/character options of 3e or 4e is probably halfway between #2 and it's own category.
It's interesting to see what people's reasons are. Personally, I think #2 is a really good reason, #4 is perfectly reasonable, #1 is silly, and #3 is an excuse more than a reason. There aren't many 5e books; almost all the non-crunch material in prior editions is compatible; and 5e is very easy to house rule if you want to convert a feat, spell, magic item, etc. Monsters are the one thing I'm really hoping we'll get official updates on. (Ie, if someone doesn't like it, they don't like it, and there isn't really a need for an excuse--though reasons can be informative.)
I was making predictions of the game's popularity during the playtest, so part of the reason for this thread was to see how close I was.
Strangely, I must admit that I'm not yet sure how accurate my predictions were. The sales numbers are extremely good, and still going strong a year later--which is important. But it seems like most people online don't know many people in person who play 5e. My in-person experience is also similar. I was invited to play in a Starter Set game with some people, but other than that I'm pretty much the 5e guy around. When people are playing it it is because I'm DMing it--although they all seem to have enjoyed it and no one has asked me to run a different edition instead.
So it makes me wonder how the demographics are breaking down. I correctly guessed that fans of D&D in general would adopt it, as well as going over well with pre-3e fans. But I had also expected a lot of non-diehard 3.5e/Pathfinder players to adopt it (perhaps "adopt" is a better term than "switch to"). The anecdotes don't appear to be supporting that prediction. It might be reason #4 that is holding that group back. I think what howandwhy99 said about not wanting to lose particular things one has is a bigger deal with the 3e-4e crowd than the older edition fans, and that may also play into it. One thought that occurs to me is that perhaps there is a lot higher percentage of die-hards in the 3e/Pathfinder fan-base than I had expected.
Again, thanks for the participation, and feel free to continue sharing thoughts.