It's never as simple as just one thing when it comes to something like play calling.
OL, Milton's lack of running, No clear "I'm going by you" deep threat like Hyatt, and a lack of accuracy when Milton does go deep
Milton has great arm talent, but his timing has never been great. And of all the things a QB can really do to separate themselves timing is king imo. It's why most the top NFL guys were able to play as long as they did (Brady, Peyton, Brees, etc.) they had timing, so even if they couldn't launch the ball 50+ in the air anymore it didn't matter.
Milton seems to always be late on lead the WR routes (crosses, slants, etc.) and that's the most timing driven route. Hendo hit those so much better and we'd see Hyatt snag in stride and his speed let him take a 7 yard slant to the endzone because of it.