LC is entirely different than every other tier. It plays different and the EVs are spread out differently. You can't use the same "oh 180 evs in defense, and 60 in attack and the rest in speed" forumula like you can in OU et al. You have to distribute EVs based on the number that the base stat ends in, which is rather complicated at the beginning if you aren't sure what you're doing.
LC is also much more fast paced and certain items are less useful which can drive some players away. Leftovers is an item you will almost never ever see in LC same with items like Choice Band or Specs. Speed is the most important stat and offensive power comes second. Oran Berries and Life Orbs and Choice Scarves are the most common items seen with probably Evolite Stones (or whatever it's called) fourth.
As Anti mentioned as well, the playerbase is minuscule at best. Only about a handful of good players really take LC seriously (it was all I played when B/W first came out on PO) and the rest of the people play it for fun and don't really give a damn about it. Little Cup also has a reputation as being a tier for people who can't play OU, so being good at LC almost comes with a negative connotation. It's got the potential to grow into a big metagame, but there are a lot of driving factors that keep many players at bay.