[zaptheimpaler]

Actually the biggest reason is RH was the first to offer zero commission trading and a very simple UI. That led to a big customer base of new traders. Lots of free marketing by those newcomers on social media and the inertia of switching brokerages kept the flywheel going. This incident was bad enough to make people get over the inertia of switching, others now offer zero commission trading, so RH is going to bleed customers as a result.

The other apps may not be quite as easy to use for someone who has never traded before, but the basics of trading and options have been democratized enough that it is probably no longer enough of a moat.