As I understand it, it is because each bishop has ‘autonomy’. Bishops are installed into their bishoprics as autonomous shepherds to be leaders themselves and to report to the Pope on how they are serving their people. IOW, the Pope is not like a King who can ‘remove’ an erring Earl, Duke, Count, etc. ‘for cause’ or ‘at will’. It takes something extremely serious for a Pope to even consider the idea of having–for the good of Christendom–to discipline or investigate a given bishop, and it certainly would not be done without the input of the bishop, other bishops, etc.
The Pope is not a micromanager. So long as the bishop does not come right out and teach, on his own authority as a bishop, that Christ was not divine, or ordain women etc., so long as there is enough ambiguity, or even outright lying (the Holy Father is not omniscent) then there is no ‘reason’ for the bishop’s actions to be called into question. After all, Jesus Himself did not remove Judas, did He? And as Scripture says, we are called to ‘forgive’ 70 x 7. I am sure that on their ad limina visits (which each bishop must make I think every 5 years or so to report to the Pope personally), the Pope has had some ‘questions’ to some individual bishops, and perhaps even some (private) admonitions and some expectations for penance and reform etc. And then it is up to the bishop to implement the Pope’s desires (in obedience) or to continue in disobedience and ‘ask for forgiveness’ the next time around.
There are a LOT of bishops who are facing a lot of difficult situations and who deal in different ways. Some might ‘appear’ to be ‘letting’ some questionable practices go on. . .but when it comes right down to it, no matter how ‘wrong’ liturgical dance might be, it is not quite so serious as, for example, pushing for women priests or gay marriage. So that ‘darned bishop’ who looks as though he never ‘cares’ about his priests fooling around with ‘weird’ extras in the Mass might be saving all his authority and his ‘big guns’ going after (quietly and behind the scenes) those ‘bigger’ apples.
We should be praying for our bishops more and second-guessing them less.