Well, there are a few main reasons for this, while none are confirmed, they are easily interpreted.
One is obviously the addition of Gigantimax and Dynamax Pokemon. While Mega Evolved Pokemon can't Dynamax/Gigantimax, it would be too OP if you could Dynamax/Gigantimax one Pokemon per match and Mega Evolve a Pokemon as well.
Another reason that Mega Evolution was removed was because it was the special part of Gen 6. Gen 7 had Z Moves, Gen 8 had Max Raids and Dynamaxing/Gigantimaxing, Gen 6 had Mega Evolution. Im guessing Gen 7 had Mega Evos as well because it wasn't as OP to use a Z Move and a Mega, also, Mega Evolution was a well-liked feature of the games (from what I've seen).
The last reason could be software limitations/a time crunch. Just like not including the full Pokedex into the game, Game Freak might've left out Mega Evolution due to not having the proper time to include it, but it could've been due to software limitations (pretty unlikely).
Hope this helps! :)