I do apologize if this is the wrong forum, but it seemed most appropriate. I was wondering why the difference in sizing for road vs mountain bikes. For instance, I ride a 50cm (21") road bike frame, but I am supposed to ride a 15" mountain bike (38cm). Why the difference in fitting?

I am asking because I have seen an old (early 90's) rigid mountain bike for sale that I would like to convert into a hybrid sort of bike, but the frame is a 17.5" (which in theory would be too big). If using for road/path riding purposes, would this frame still be too big?

No real answer here to give you. I say that because sizing is often different between manufacturers of MTB's. Moving from MTB to road I fele the MTB world has more sizing variation. At least that is my perception. For example, Trek Remedy 2016 9 29er frame size is a 19 but fits like an 18! Hugh? Now it gets worse. I loved the AL Remedy so much I wanted the 17 carbon version. I bought a frame only. Frame size was different from the previous year. Sooo confusing. It's like trying to keep abreast of axle standards on MTB wheels. Insane. Some fit bigger and some fit smaller even withing their own product line.

Going back decades like you are is probably a lot harder. Maybe you will get lucky and find someone on the forum who has a wealth of experience. Have you tried to find any old info from the manufacturer? Sometimes there are threads on forms dedicated to a specific out of date bike.

Thank you anyhow. I figured that would probably be the answer. However, I was more curious in a generic sense of why MTB frames tend to be on the "smaller" side. Is it just the way they measure?

One reason: the bb is higher so the seat tube is shorter.

MTB bikes require more stand-over clearance as one is more active in the saddle. A road bike just needs to clear the jewels. That's my take.
If you want to make a road bike out of a MTB, then my all means, get the larger one.

Actually, it does not.

The BB thing makes sense to me, especially with those older style MTB that had much more similar geometry to a road bike than the newer geometry MTBs.