4e was designed around having a simple math progression and monster roles.The systems in 4e worked well. I wasn't a fan of that edition as a whole, but that bit of it (and especially the encounter building systems) were highlights.
5e wasn't.
That's 75% of the problem.
5e math is based on feels of the game aspects then the math curve was plotted after. That's why so much of the MM didnt follow the rules.