I see no real reason not to have 56x 32x 56x or even 60x 32x 60x drives.
Well, the reason isn't the drive, it's the discs. I believe many of the disc makers won't support 56x and larger. IIRC, the reason is that the current factories can't make the discs flat enough for those speeds. To do so would require retooling and there aren't the profit margins in making CD-Rs to support that.
Do a good search around the net and see what happens when CDs get to 56 or 60x. The wonderous world of good ol' centrifugal force comes into play and the CDs explode due to the forces on them. The flatness also means that spinning at that speed, you really don't want ripples in the surface so the disc acts as a wing.
But, one question that I've pondered is what is the end speed for CD-RW? I know the 32s are coming soon, but is there a limit for the RW technology? Will we ever see 52x52x52?