For example, you have to maintain a fraction of your total deposits in a cash account. That means that if RH becomes a wild success story, they may have to keep millions or even billions worth of cash stack.
I believe there are many other requirements that are not easy to satisfy when you're a small nimble startup.
Personally I hope the banking industry in the US gets disrupted, right now it's a dinosaur and consumers suffer.
However, with the current legal framework that doesn't seem easy.
Seems like RobinHood was trying to disrupt banks and then the big boys (SIPC) shut them down fast.