The problem is that most of the troubled mortgages aren't owned by the banks themselves. They were bundled into securities during the housing boom and sold to investors.
To reduce principal payments on those mortgages, banks often must get permission from the investors who hold the securities — and may not be willing to take less.
1 Comments in Response to Take two: Gov't tries new fix for mortgage crisis
Obama says his new mortgage plan still has problems:
http://www.mcclatchydc.com/2010/03/26/91199/new-mortgage-plan-still-has-holes.html?