One of the biggest reasons is probably the reason behind using any software as a service rather than doing it yourself: you can concentrate on what you do best and not spend time managing things yourself. A couple of our users switched from using open source alternatives for that reason. We've put a lot of time into our infrastructure and decoupling things so we're very resilient against outages so you don't have to. We've also put a lot of time into making our library more robust and well tested than any alternatives we've seen to date.
I'm guessing because it's better than what they could integrate and build up with an open-source alternative in a short amount of time and thus more cost effective.