Koopmeiners was never really given a chance, Allegri introduced him very slow and kept subbing him out.
when he failed to deliver according to expectations, he was perm-benched and never given a chance to play again.
The main reason was Locatelli, Locatelli started in much much more worse state than Koop, worse because he has no positive contribution and committed many and more crucial mistakes, but the journalists favored him, outlined always that he had good statistics, esp miles covered Tudor kept playing him no matter the cost for Juve.
Even when that cost was grave enough to cost his own seat, he has never given Koop the chance to try again and they even made Locatelli a captain, so that the next coach would have no option but to start with him, in order to keep team cohency.
Spaletti declared that he doesn't rate any of them and needs a creative midfielder.
The mngnt that imposed Locatelli and had decided that Koops will be sold, unless he accepts the subber role has imposed that.
Locatelli grew powerful enough to dictate who starts and what role each player has, held no grudge over Koops, because he never challenged him.
As a compromise, Koops was only used as an extra CB, in a place that could not compete with Cambiaso or Locatelli, that and only that.
As long as Locatelli is in charge, nothing will change