The main problem was Middleton breaking down physically. Once that happened the Bucks lost their #2 scoring option. This made the Bucks trade for Lillard. The Bucks couldn’t count on Middleton anymore and needed a #2 to pair with Giannis. Unfortunately Lillard and Giannis was a poor pairing because Giannis is not a good pick and roll player.
The next problem is the poor drafting by the Bucks. I’ll give the Bucks a pass on Parker as that was a freak injury. If Parker never gets injured wow that team would have been a great one. Middleton, Parker and Giannis was cooking that year offensively when all three were healthy and on the court together. Then Parker gets injured and then it was over. Every draft after that Parker draft was disaster after disaster. Other than Brogdon there wasn’t a pick that was a clear positive.