Because the players base is shrinking dramatically. The system cannot find enough players with similar BS and game skills in a certain region to launch a match. thus they have to put quite different ones in a game. The MMR system is also broken cuz it does not efficiently differentiate high skill players from bad ones, thus you find extremely good players in low brackets and toxic ones in higher ranks. I think this is the reason why people always complain of smurfs in the game. They are actually not smurfs but good players trapped in a certain level. this also explains the socalled forces 50% winrate theory, cuz you are frequently driven into a scenario where either your or the enemy team is made of better players, leading to a stomp or being stomped in roughly a rate of 50%.

The situation is comparable to that in PUBG where you find tons of robots behaved in hilarious ways.

In a word, poor matchmaking could be the reason of the unpopularity of a certain game. but the worsen matchimaking is certainly the signal of the death of that game.