The stated reason is total bunk. But "we know" middle-class voters care about their net equity and the value of their home so let's argue that a foreclosure moratorium would negatively impact property values. And the mechanism by which we argue this happens doesn't matter because in narrative logic all that people will remember is halting foreclosures is bad for property values.

By laying out pros and cons we risk inducing people to join the debate, and losing control of a process that only we fully understand. - Alan Greenspan