Because most of their depositors were business that kept >$250K (the insurance limit) in the bank.

Why would someone keep more than $250K in bank deposits? Isn't it more prudent (looking back no more than 15 years) to keep T-Bills or some other liquid security that isn't subject to a bank run?

$250k isn't much for a company with employees

you have to keep some cash somewhere to meet short term expenses (like payroll)

Is it not possible to meet short term expenses with a short term loan which is discharged over a few days by selling liquid assets? Maybe banks just don't want to let you do that.

It is, and no prudent company should have lost their entire war chest unless their funding arrived just yesterday. But when you're spending $10M or $100M a year, it's operationally challenging to make sure you never have more than $250K in the bank while you're doing it. (It's much, much better to be imprudent and have too much in bank deposits than miss payroll because you have too little.)

Because the insurance is for $250k and most depositors had much more than that.