I still don't understand why people hate Messenger so much. It works fine for me, indistinguishable from when it was included in the main Facebook app.
Messenger is much more intrusive to the whole phone experience than the Facebook messaging functionality that was included in the app. Though, to be honest, even that went downhill in the same direction as Messenger -- though not as far -- before it was removed and messenger made mandatory.
Many don't trust FB anymore or the invasive data tracking of Messenger. I've overheard comments from friends and family that they won't be forced to download another app, especially one that keeps such close watch on users.