The problem with IPA's is not that people enjoy them or that brewers brew them and pubs stock them. The problem is that there's so many of them, all purporting to be slightly different, that it's pushing out other kinds of beer. The amount of times I have walked into a pub and there's somewhere between three and seven IPA's and I ask if they've got any stouts and the answer is, "We've got Guinness."
Part of that is seasonal. You're not going to get a stout in the summer, just like you're not going to get an Oktoberfest in the spring or a Shandy in the winter. Like you said about IPAs, there are so many variations, they've become a year round beer. Other year round beers are things like lagers and ales. Comparing a seasonal style to a year round beer isn't a fair comparison
If you can't find a stout on tap in the winter, you're going to the wrong breweries
I live in the UK. My experience has been that the selection of beers isn't that seasonal. Sure there will be guest beers in some pubs/chains but for example I can go to a Sam Smith's pub and they have four stouts year round plus a pretty good porter. Just a lot of people don't like to go to Sam Smith's pubs because the owner has... feelings about one thing and another.
So if I'm meeting friends usually there's something nearby that we're planning to do and in a great number of pubs the choice is Guinness or nothing.