There are a lot of industries that have niche software needs. It’s hard for a competitor to break in because the market is only so big and it’s better to have something standard and time tested.
Interoperability is often limited to a one-time database migration, and often requires a specialist to do a lot of the transfer manually.
I don’t know if that’s the case with this software because it’s not my industry, but I’ve dealt with similar issues. You’d be surprised how much of the world still runs on AS/400