PWInsider.com is reporting that the reason Bret Hart is not being used on WWE TV right now is because his contract with the company has actually expired, and rather than offer him an extension, they are using him on a per-appearance basis when they want him to appear on house show events.
Another reason he's not on TV is because WWE has made the decision not to use him for low-level appearances and/or cameos. When Bret does appear on TV, it will be when WWE feels they have something "major" for him. He has, however, appeared in such markets as Calgary and Europe as he still has a very large following there and has more of a place at those events. It's being said that the relationship between Hart and WWE remains very good.