Probably because I've seen/heard/read about guys not being locked in and having sloppy weeks of practice (several members of the 2007 squad off the top of my head) and then still playing on Sunday. I've seen/heard/read about guys coming off of injury or just acquired (Martellus Bennett) mid-week that played on Sunday.
Now, to follow up that reasoning, I'm having a hard time envisioning what Butler could have done OUTSIDE not being locked in to warrant benching him for more than a quarter, going into a defense that you don't have the horses to run (particularly against an offense like Philly) and sticking with it, and essentially flushing another Super Bowl down the toilet. Even a modestly better defensive effort - which isn't beyond the realm of possibility once you get McCourty and Chung back to doing what they do, Butler on the field, and Bademosi and Richards OFF of it - would have won that game with the way the offense played.
Mayo is just the most recent to weigh in on this. He joins a host of others that say they never got an explanation about it. I would hope Bill gives the guys one before the season. There is some air to clear there.