I have received a few iTunes notices that there was a problem with a TV episode and to download a fresh copy. So, I suppose that a conversion of a large library like HBO's takes a while if they are redoing it optimized for a different service, then to have to QA it all....
I also suspect that the new services also have to have local servers scattered around the country to balance the loads and provide less lag, the library has to be copied to every one of them and updated regularly.
Perhaps they are hoping to further divide the load up with the three different services.