You mean why has McDaniels withdrawn himself from previous Head Coaching searches even though he's been considered THE top coaching candidate?
You'd have to ask him - what's not debatable is that that as long as he remains with the Patriots he'll remain among the most highly sought coaches. McDaniels is currently being mentioned as a top candidate for the Colts, Rams, Giants and potentially a few other teams.
Top NFL coaching candidates: Josh McDaniels highlights rich field
A team like the Browns probably doesn't even bother asking him (heck - they'd LOVE to have his 11-17 head coaching record! They'd probably give him a big extension with that "success"!)
I'm sure part of the reason he pulled out of the 49ers search was that he learned his lesson in Denver and won't accept a spot where he's being set up to fail or doesn't have full control.
That being the case he can wait until the perfect offer/situation comes around OR he can make HC money while enjoying the lower stress of an OC position until Belichick retires (assuming he has that offer from the Pats.)