Bank accounts are very sticky and most people don't move them in search of better rates. In particular, people are not thinking of cash sitting in a checking account as anything other than short term cash they don't expect to earn a return on anyway, so you wouldn't change your entire commercial bank (that everyone else in your industry also uses and expects) just to chase some small returns.
SVB was holding so many uninsured deposits because they were the operating cash for businesses. If you have $100M, the difference in interest would be $3M a year between 1% and 4%. What kind of business wouldn't be paying attention to that?
