He had done that in the past bc he relied on Manu, and co. to hold the bench together as Manu had consistently been one of the team's best players and had not ever needed a starter to hold his hand with his bench group. This time, this year he did. Last season Manu was done too.
ElNono will come out of the woods and shoot this down with a stat here or there, but he knows deep down Manu's confidence was shaken in the latter part of the season and he didn't arrive to the postseason playing his best. Marco and Patty were the ones holding the bench with some insane hot shooting and Diaw had a good series scoring at least, but Manu was subpar and it looks like he's now done and as Manu goes so goes the bench.
Pop should have realized these were not even the early season 2015 Spurs. But he had relied on these guys and they have brought him 5 rings... just no more. Time for him to recognize that too.