The people are stubborn and many have had experiences with subpar enterprise software. If they are billing clients, any additional step in software is akin to a national tragedy to them. They will complain something is "broken" if it doesn't fit their mental model of what a software should be before trying anything new.
Another thing to consider: we are conditioned to believe that the web browser is a "universal runtime", i.e. it's the thing that everyone has, so you should target it. But actually, it's not the only one. When considering Windows computers (which is all that matters in offices), Excel is a kind of dark-matter universal runtime, for dark-matter IT[1]. Better, it's one that doesn't require a persistent server, or logins, and it's peer-to-peer, in a sense. Everyone has Excel installed, everyone has an org email account, therefore everyone can email xlsm files back and forth, no extra setup required (try doing that with exes). No SaaS offering has that kind of caveman convenience. And you might think -- that's horrifying. But email is the way communication/collaboration actually happens in the median bureaucracy, so Excel's ubiquity and file-orientedness is an insurmountable strength.
> This sounds like an assumption worth validating, because it sounds like an easy product to sell for lots of money.
Unfortunately, it's not so simple. I've worked in exactly this field, getting such institutions/companies to adopt this kind of software is ridiculously hard.