Because that's what Hardball Talk's Matthew Pouliot suggests..
We tried to tell you that Ausmus interviewed for the manager position, but McTaggart said we were no-good, dirty liars, and then Ken Rosenthal came to the rescue, and the day was saved. Dogs rolled over on their backs and there were tummy-scratches available for everybody. Regardless, Ausmus took his name out of the running to become the next manager of the Astros.
The line of thinking is that Ausmus, a Connecticut native, was tipped off that the Red Sox wouldn't retain Bobby Valentine (though Deputy Street pointed out on Twitter that EVERYBODY knew Valentine was getting canned). It makes sense. Ausmus presumably grew up a Red Sox fan. And when you have a chance to manage the Red Sox, you have to take it. (Is that still true?)