You have to understand the situation before the release of the game: Every COD before it(Infinite Warfare, WW2, Black Ops 4) was a flop in some sense. MW2019 was not only going back to it's roots, the game was actually comparable to the oldschool MW titles in quality as well. Hell, they even removed the loot crates that plagued these titles...only to replace them with 24$ "micro" transactions, yeah that started here.
Oh well, at least the Multiplayer was nice and fresh...when it wasn't making you sweat, since this is when the SBMM as we know it today was activated, it was much more visible in future games however.
Anyways, that was the hype, the reason some people hate MW2019, aside from the ones listed above, is because it was slower and more methodical. That's it, they didn't like that they took away 3 lane maps and had players taking buildings and waiting behind doors, because everything has to be chaotic ADHD riddled sliding and bunnyhopping fest like Nuketown or Shipment. If someone hates MW2019 it's either because they sucked at it(ie can only play COD one way and cry when the game changes it's meta) or because it introduced us to the cancer that is modern day COD. That includes Warzone BTW, I know I dropped out once that became a thing because I knew it would consume not just the entire game but the IP as a whole.