I/O is what the customer chooses the vast majority of the time. When a salesman relizes what his big seller is he pushes it.
Granted most people selling these products have no knowledge of them beside what products sell but that is another discussion.
Also, the reason lenders are pushing conforming loans is because they do not know if anything else will sell on the secondary market and do not want to get stuck with a loan. So the incentive is to stay in business.
