It takes time for new technology to be adopted for this. Keep in mind the stakes, the extremely difficult requirements, and the expense of testing a new system.
Imagine trying to update the system to use a different technology. Then imagine how amazingly bad it would be if it didn't work as well as the old system. Or if it lost data when it shouldn't have. And so on. It's the classic system rewrite conundrum. You can rewrite a system at fairly low cost, but in order to bring that system up to feature and reliability parity you need to put in an order of magnitude more effort. And in this case you don't really want to deal with "early adopter risk" so you need to dump all that effort (read: money) in ahead of time.
Imagine trying to update the system to use a different technology. Then imagine how amazingly bad it would be if it didn't work as well as the old system. Or if it lost data when it shouldn't have. And so on. It's the classic system rewrite conundrum. You can rewrite a system at fairly low cost, but in order to bring that system up to feature and reliability parity you need to put in an order of magnitude more effort. And in this case you don't really want to deal with "early adopter risk" so you need to dump all that effort (read: money) in ahead of time.