First, HIPAA requirements are in part IT requirements. Every single hospital works really hard to comply with these, because there are huge fines if they don't. Some of the problems with hospital IT security might be due to defects in the already quite onerous HIPAA specification.
The second is that hospitals are extremely low-margin institutions. Most hospitals (even the really big ones) just break even, especially if they're teaching hospitals or serve poorer areas. IT security doesn't really produce any revenue.
I agree it's a problem. This needs a systemic solution–who would pay for IT fixes? Reimbursement is declining and government payment sucks. Most hospitals are in crisis mode as it is.