It’s also about the barrier of entry. Discord has a barrier, but it is not high. The fact that this platform is their commercial product ensures that issues are minimal (well…). For non tech-savvy people and those who don’t have time / don’t want to put in time to troubleshoot and figure things out this is the reason they go for these commercial solutions.
Just my observations from trying to switch friends and family over to free open source alternatives. It was a huge hassle to even get people to try out and move to signal, but signal has lowered the barrier quite a bit and now it’s stable (was not always as fast and stable as now).