Our reading of more recent data is that they have been a bit stronger, but nonetheless consumer confidence remains low, the housing and labour markets remain weak, and the repair of household balance sheets still has quite a way to run. BIS Review 153/2010  1  Basically, the US household sector is in very poor shape.
First, because the principal is paid down in the case of principal-and-interest loans, those loans are likely to be less risky for the banks; other things equal, you would expect them to attract a lower interest rate. Second, the banks have added a premium to interest-only loans of late to encourage customers to take on principal-and-interest loans and constrain the growth of interest-only lending.