T-bills are short duration (up to 1 year). If they had stuck with T-bills they'd be in better shape today.

SVB reportedly bought longer-duration US Treasury debt which is more sensitive; one source says their portfolio had an average duration of around 5.7 years, up from 4 years a bit over a year ago: http://twitter.com/MacroAlf/status/1634626136364679168

The issue with longer-term debt is that if you need the money now, you can't force the issuer to redeem it before the bond matures - instead, you have to find someone else to buy the bond it from you. And if it's paying half the interest that a new issue would pay, nobody's going to give you face value.