I know a few people (including my wife) that will not use anything else except Robinhood, simply because everything else seems too complex. All they want to see is a basic chart and couple buttons.
Such extreme simplicity is what made iPhone the leader (first phone with one or no buttons?), it is what makes Craigslist so popular, and it is the requirement for any modern apps. Who cares about 20 products or websites or apps doing the same thing when people always come to the one that's simplest/easiest to understand, and that they already got used to. People don't even care about the price of products like iPhone or Tesla, it's all about usability, brand, familiarity and what their friends use. Professional traders may not use Robinhood, but how many professional traders are there, within 6 million Robinhood users?