Fair question, although the basic answer should be obvious: the users still need access to the data! So, the question becomes more like "why not upgrade", or more specifically "why not migrate the data to something that is not so shockingly obsolete", since it's probably clear that there is no real upgrade path here (both SCO and the vendor of the accounting system are long gone).

Usually with systems of this vintage, "just dump all the data to Excel or PDF and get it over with" is a good option, but in this case both the volume (with the requirement to run queries on it) and the limited options available for export (the system can only print predefined reports, and they don't contain everything required for filtering) prohibit that.

So, next stop would usually be "reverse engineer the application data format and convert it", but the unholy collection of binary files used by the accounting software here has defied analysis: it's not Btrieve or MS-ISAM (popular semi-database formats for COBOL and BASIC apps of the time), and decompiling the binaries only yielded some generated-by-another-set-of-tools braindamage that didn't clarify anything either.

The choice then became spending huge amounts of money, or wallpapering over the tirefire and keeping it running. Unsurprisingly, the outcome was the latter, which is perfectly OK in this case, as the system is not exactly load-bearing, and actually sort of fun to maintain.