I don't have a Robinhood account, but presumably the fact that Robinhood's free trading was what forced all its competitors to eliminate commissions means that lots of people had accounts there before that and kept them due to inertia.

Compared to Schwab and Fidelity, Robinhood still acts mobile first, as opposed to a desktop site condensed for a phone. It's not a good reason to use it for something like this, but the experience is much more fluid and pleasant.