When you've got musicians the calibre of 5o5, 2 tracks are sometimes enough :0)
Supporting >4 channels might be an impressive tick in a check box on a list of features, but game designers mostly just want the sound side of things to be as lightweight and efficient as possible and "sufficient" is usually good enough. Having the effects supported 100% is also more desirable than just a bit more of everything so that's what most will vote for before extending capabilities beyond what the musicians are already producing.
When I used 16+ channel trackers before on pretty restricted 20 year old hardware, the compromise on sound quality wasn't exactly massive but it gobbled up too much resources. I'd hate to think what would be left of the Jaguar playing 16+ channels of music plus in-game SFX. It wouldn't suit every game that's for sure :0)