I'll put money on out of date OS and lack of budget for any kind of upgrades.
I've just filtered my incoming bug reports by people still running Windows XP. Almost all of them are pharmacies. You tell them to upgrade, and that they're not PCI-DSS compliant. For 9 years.
The rest of the NHS is likely even worse.
Sad thing about IT security, it's one of those things that costs money and you don't see the benefits. Only the catastrophic failures.
Chances are they use some internal sites that only work in IE. Could even be using some Java Applet or ActiveX plugin that hasn't been patched in 15 years.
Even Chrome abandoned everything pre-Windows 10 earlier this year. I reckon there's more machines than not that couldn't upgrade OS even if they wanted to.
It's a lot multiple factors, opportunity cost, increase of online users due to pandemic, home from work, but biggest one being companies/governments don't care about customer/user data so they they do bare minimum required by law to secure it. Which makes them an easy target. Getting breached is just part of doing business and a lot private companies just pay ransomware and go back to normal business.
I would have thought the Information Commissioners Office UK, would have the power to issue hefty fines. Mind it sounds like those may the consequence of doing business as well. Depressing.