Cause they feel the need to make themselves known.
no, it's because they've been on this forums long before the pony show came along. they know the forum, they're familiar with it, they don't want to go elsewhere (or if they do go elsewhere, they continue to use this place as well)
one reason the pony topic is so high in page numbers is because it has become a form of off-topic itself. it's at the point where the same users come and go and post about essentially everything, but generally try (or at least i hope they try) to keep it pony-related. there is a lot of pony-related content, and although it is currently slow it booms when the show is on-going (it is currently waiting on the next season).
personally, i also go elsewhere to discuss the show and whatnot. but i started out here, learned about it here, saw it first here, played blockland long before the show was even conceived. i use the topic here because for one, as i said i know the guys who are constantly go in the topic and post so i have a strong familiarity with it, and secondly i've never found an alternative that had the same bulletin board style that i could actually get into.
