Recently submitted the Forza Motorsport 3 Ultimate Collection to the Mazda RX7 game group, but it was rejected on account of the Ultimate Collection being a compilation.

Not bothered by it being rejected, but I am wondering why that compilations are not included in games groups?

It's because they clutter the overview. The individual entry will have it.

Compilations with exclusive content are still allowed, right?

Yes

That was quick!