They all do it. The NFL is as much a good 'ole boys network as the plantation owners of the south. I think its because of trust, they have to trust the guy implicitly and most will choose people they are already familiar with. MM hasn't done the best job on his assistants, but its not that different from all the other super- awesome- smarter- better coaches around the league.
What MM did differently is to give Zook a year to learn the personnel, schemes and organization so he could hit the ground running.
If you're gonna go "crony", this is the way to do it.
My speculation is that previous ST assistant Chad Morton told MM that "Slocum sucked" and MM sent Chad packing- but not without considering that perhaps ...Slocum sucked.
In comes Zook, a trusted friend who confirms Morton's allegations, and then Slocum is summarily dumped.