Its to keep the pony discussion in one place.
The fast paced cycling of content that these "fast" threads offers us is nice. Everyday I can go to off topic and see a truly random selection of things that I may or may not want to discuss, and if I don't like them, they'll probably be gone in a day or two at most.
But to see that there are threads such as the MLP thread, which is just a few bronies talking about the show and socializing, as a permanant part of the forum, something that will always be in the top 10 threads in off topic (as well as the largest thread ever) kills the idea of a random, fast paced off topic, with a constatly refreshed set of topics to discuss, for everyone but the handful of people who have emassed probably
hundreds thousands of posts each in that one topic.