At some point in the 1970s or 1980s, Ticketmaster made the extremely unusual decision of writing their own operating system for VAX. Even back then, that was a pretty weird decision. And it wasn't Terry's idea, they started doing it before he worked there.
https://news.ycombinator.com/item?id=9808995
And apparently decades later their backend was still stuck on it (is it still stuck on it???), albeit now running under a software emulator instead of physical VAX hardware.
Most people on VAX ran either Unix or VMS, both of which offered migration paths to other platforms (Alpha -> Itanium -> x86 for VMS; and there were lots of alternative hardware platforms for Unix). But Ticketmaster's weird decision to write their own OS made that almost impossible.