1) No one has the time for a super-long diplo all in one sitting 2) More people switching over to strategic games (1v1, 2v2, 3v3, etc...) 3) Booting - strategic or due to players not playing - kills the game quickly 4) Why play a RT diplo when you can play a multi-day diplo?
1) 'Good' diplo players moving to clans and invite-only games. 2) WarLight losing popularity in general. 3) They really get tedious after 20th game with half players booted in turn 3, four PE's, no fun roleplay and general noobiness.
I know I used to only play real-time games when I was a noob, mostly RoR and diplos. But it is so much simpler to play multi day games, where I can take my turn whenever and not worry about boots. Like others said, it is not very much fun when the USA, China, and Russia get booted from a real-time. And noobs are the primary players of them. Multi-day are annoying because you always have to be checking chat and you can get them mixed up and stuff, but overall they are still better.
I think it's mostly to do with the number of booters, I tried to host one last week, and I think about 3-4 people made it past the 3rd turn, waste of time. I tend to just host invite only Multi-day Diplos now, and occasionally run public Multi-days to find new good Diplo players, but that's about it now. And from the public ones I maybe only find 2 or 3 new good regular players.
I think Diplos have just had their day, people are getting more into the Strat team games now I think...