To throw in another reason why old versions are still popular, a good chunk of modded players use modpacks, and some of the more popular ones are pretty highly

To throw in another reason why old versions are still popular, a good chunk of modded players use modpacks, and some of the more popular ones are pretty highly polished collections of mods with a bunch of glue to make them work together as a somewhat unified experience. This can't really happen until the mod ecosystem for a given version has already stabilised a fair amount (and can only happen on versions for which a large number of mods are available, which tends to be every 5 releases or so).