Lack of geared players, really. We've got a few guilds running in-house ranked on an almost daily basis now, so hopefully as we get more players that are semi-geared and capable of standing up against the full WH guilds on the Republic side we'll be able to start partaking in regular ranked. Sure, you get more in comms for a loss in ranked than you do for a victory in regular but it's pretty hard to keep a group of 8 players together when they're losing constantly.
For PUGs - It also seems a lot of imps have a severe lack of morale, and the in-fighting gets kind of ridiculous sometimes. Lot of disrespectful players on our side of the field, which damages unity. On the plus side with F2P there's a lot of new players on both sides of the field, so Imps actually win on occasion now. Might help alleviate the gear problem.
These are just my personal observations, others may feel differently.
We have someone in our guild trying to organize at least one ranked team to run. I'll post on here if things work out and he gets together a solid schedule for when they'll be doing their thing. I think I saw a couple others post on here. I imagine over the next couple months you'll start to see a bit more of an Imperial presence in the ranked warzones.