Foreclosing would require that the mortgage owners re-estimate downward the value of the second liens to what Morgenson calls "fantasy levels." That, in turn, would require them to admit that the loans had gone bad and, therefore, require that they repurchase the loans from Fannie Mae and Freddie Mac.
I imagine the grand thinking behind all of this, to the extent that it exists, is that if you just kick the problem down the road far enough a recovering economy will start to magically solve the problem. But the recovery isn't happening, so...