There was a big exodus to signal for a while a couple years ago when meta were fucking with their whatsapp privacy policy, similar to the exodus from reddit to lemmy.

Having your infrastructure on a cloud provider allows you to keep your costs in line with your current amount of users, if you have a big influx you can immediately scale up to accommodate them, and then when that spike in users dies off as they invariably do you can scale back down instead of being left with a load of hardware you’ve just bought for your new users (that have since fucked off) and now aren’t using