That would at the very least be unpopular with their customers, if not outright illegal. What would it look like to be a customer at a bank that did that? Especially for something like SVB - your startup closes its funding round, they wire the money to your bank account, and the bank refuses it?
SVB lobbied for a law that changed a crucial regulatory threshold from $50 billion to $250 billion. From ~2016 to ~2020 their assets jumped from <$50 billion to >$200 billion, with most of that happening from 2020 to 2022.
SVB was perfectly capable of restricting the total amount of assets it held. It wasn't a matter of rejecting small deposits; it was a matter of allowing (or rather, soliciting) huge clients to open new accounts to hold hundreds of millions or billions in assets.
