Actually quite a few slow loading drives take mini cds as long asyou load them in the middle of the drive.. take the wii for example, takes gamecube games (much to my horror when my friend put one in his wii and i almost did a action-slow-motion-dive to stop him becuase i thought it wouldn't work).
I'd actually try to put a mini cd in my mbp if i can find one here, I have some from one of my trips to Japan.
In regards to why they're not in the PC market, my $0.02 is that you'll notice that in the PC industry, they're always going for the fastest, latest and greatest, and slot-loading dvdrw (for example) are now just reaching 8x in a quantity/quality worth while for mass production in notebooks, whereas tray loading drives with the same speed have been around for quite some time now.
In the competitive PC industry, if you dont have the latest, everyone else will run away.. in that light, Apple is in a unique position as they are both the original (and only) direct sources of hardware (in the form of computers) and software (in the form of operating systems) so they can do really, whatever they want!