The inexperience is huge, teams don't want to take a chance on someone that has little Coordinator experience to lead half a team, let alone HC experience to lead an entire team to what they want them to become.
Also, McDaniel's personality is very unconventional to say the least, with how much he jokes, and takes things lightly, acts like one of the guys in the offseason, and friends with most everyone on the team. Most teams see a HC candidate more as tough as nails, no expression hardly ever shown, serious from when they get in, to when they're eating, or in the bathroom, to when they leave.
The thing to consider about McDaniel, he is very smart, and though he is very friendly, and takes things very lightly, when it comes to football and the season, he can be pretty demanding and very serious about it...Either you take your job very seriously, or someone else will do it. He expects 100% effort, nothing less, and if there is an aspect of your game you are not good at, you will work hard to correct it.
Many don't know what McDaniel is about, including us to a lesser degree, considering we have yet to see what he can do as a HC, but that is also how you find an above average HC, you have to take a chance with what little you do know.
Bill Belichick was always thought to be just a Parcells coach that had success because of Parcells, until you notice that Parcells without Belichick never was really very successful, yet Belichick without Parcells proved to be extremely successful...Even with the Browns.
Player coaches can be extremely successful like Joe Gibb, Dick Vermeil, Bill Walsh, Andy Reid to name a few, as long as they are Brilliant coaching at least one side of the team (
for McDaniels here), are organized and calculating enough continue to bring new aspects opponents have not seen or prepared for (Another
here for Mc-D) , is able to be friends with players, but can and will discipline or release players that disrupt or decide not to listen (We will see here, if during season someone becomes a problem), and finally can make changes when game plan is not working (Another we will see).
I like that nobody has as much faith in him as we do, because if he's the real deal, by the time they figure out how good he is, it will be too late.