The Q45 was their first vehicle, in ~1989. As far as I can tell, their second car was the M30, followed by the J30. Those were different (size) classes, so, historically, they've always used different letters to designate different models. While the QX4 (SUV) came out in, what, the late 90s, and ALSO began with a Q, it's never been a historical convention to just name all of their vehicles with a Q. There were only 2 "Q" vehicles, and one was actually a "QX".

I suppose you can call the Q superfluous, and then point to BMW if you want another company that just names cars by number, but at least there's a well-understood history and progression between the 3 series and the 5 series and the 7 series, but I have no idea what makes a Q60 (big number!) a sporty coupe that used to be a G35/G37 and now the QX30 is a small SUV-like-thing, that's, I guess, the size of older G35s but that means the Q60 somehow jumped 3 places in the tens column for no apparent reason.