Why are there manufacturer's with only one car?

[ManteoMax]

All sorts of considerations, including what makes sense for the game, time and cost budgets, logistics etc. Here’s a developer comment on adding cars:

@JONK1969

@lanciastratos73

I highly doubt T10 even looks at the wishlist at this point with the thousands of requests it has. It’s a valid question to ask because it pertains to licencing more over requesting specifics cars .

We do look at the wish list, very closely in fact, and actively take that into consideration when selecting cars. The Ferrari 488 we just released was one of the top 25 most requested cars on this forum (top 3 in fact).

If we did add SAAB again, it’s not because they are most requested cars in the wish list - they aren’t by a long shot - it’s because they are iconic cars that deserve a spot in Forza (especially the awesome 900 Turbo SPG). And we’ve been trying for a while to make that happen. Unfortunately, car manufacturers or the agencies who represent them sometimes may decline to participate for one reason or another.

If you did some light research on SAAB, you’d know they’ve undergone a lot of ownership changes in the past decade, and video games may not be at the top of their priority list:
Saab Automobile - Wikipedia

Not much you can do except to keep trying, and we do.