Hi kt90, welcome to CDFreaks!
I think the reason is one of marketing.
CD-RW burners were sold with the burning speed as the primary marketing parameter; people would buy an 48x burner rather than a 40x burner, a 52x burner rather than a 48x burner, a 56x burner rather than a 52x burner and so on...
The same is happening now, with 20x being preferred over 18x which is preferred over 16x and so on for DVD burners.
But the CD-R burning speed of a DVD burner is no longer used as a marketing parameter, so it's not necessary to push the writing speed of these drives to 52x or 56x for CD-R burning, and it's much harder to get a good burn at these speeds than at slower speeds, so the DVD manufacturers only offer 48x or even 40x as the highest burning speed for CD-R media, because it will usually work and people are not in general making buying decisions based on the CD-R burning speed anymore.