Yeah, then you'll have to wait for Megiddo to chime in. None of us (that I know of) were around when he released his first version.
I was always around, even for the first AI mod, even though I wasn't involved or contributing.
This brief history lesson can help answer your question. Garry's Mod v9 (the first that supported Lua scripting) was released on Oct 28, 2005- ULX beta was released 3 days later on Oct. 31 (very barebones), and an official v1.0 release was released just under 2 months later.
Now, we've completely rewritten ULX v3 from scratch for Gmod 10, and it's a lot more complex- but you can look at the timeline to see how long it took. Megiddo would definitely have more personal insight and more details.
As for motivation... well, back in the day, we had lots of free time (sans school), and it was a fun project with a good community. Nowadays, we have full time jobs, families, etc., and the community is a bit more toxic. Plus the whole advent of people making lumps of money over relatively small and simple addons, while we get little/no donations-- it's rather difficult to stay motivated. But, we still are passionate about the project- we just don't have a whole lot of available time to work on it.