Because the suppliers of IT systems (eg Microsoft, Crowdstrike) do not agree that they can be used for life-critical purposes
If someone is injured or dies because the hospital has inadequate backup processes in the event of a Windows outage, some or maybe all liability for negligence falls on those who designed the hospital that way, not the IT supplier who didn't agree to it.
If your assumptions rest on corporate entities or actual decision makers being held legally liable, then you've got a lot of legwork ahead of you to demonstrate why that's a reasonable presupposition.
If someone is injured or dies because the hospital has inadequate backup processes in the event of a Windows outage, some or maybe all liability for negligence falls on those who designed the hospital that way, not the IT supplier who didn't agree to it.