UX. Try out a few other brokerage experiences, and then try Robinhood, and you’ll see that the market was ripe for the taking for a team with design sensibilities.
Etrade made 614 million in profit in 2017. 19% of revenue was from order commissions, or roughly 449 million. So Etrade could eliminate commissions and still remain profitable. https://en.wikipedia.org/wiki/E-Trade
However, IB is decidedly not meant for the young, millennial, small amount of investment principal, getting started with investing/trading segment that Robinhood has carved out for itself.
So Robinhood's angle isn't just mobile first, it's mobile first for the segment of the population that expects such a product experience, who are going to grow their portfolios and trading activity in the coming decades.