Could the name "DB9" have come from 25-pin serial ports with only the minimum 9 pins populated? That would be a correct "DB9" and would also be valid electrically. I think I've even seen one of those in the wild before.

Interestingly they have 25 to 9-pin adapters and the majority of DB25 devices I worked with were fine either way. So, the devices generally used 9 pins or less (or at least 9 or less pins were "important").

There are, of course, devices that use more than that, but most things seemed to use less. Maybe that's part of the reason the 9-pin became more standard.