I don't think price is most people's problems. I think usually it's just

1. they don't want to depend on a third party's existence. If Slack Inc. goes under or gets sold to eBay, the team won't have Slack anymore, and

2. they want privacy. They don't want conversations being watched or sold, especially to competitors.

I'm pretty sure that's all it is. Someone correct me if I'm wrong though please.