I think there's a few factors. One is that there are too many types of cards out there, though only a few are really mainstream. Second, card readers are fairly cheap to buy. And third, I think they feel it would mess of the sleek look of their computers.
I personally would prefer a card reader in my MBP rather than a 34-pin expresscard slot. I could have made use of a card reader several times since I bought my MBP, but have yet to have any use for expresscard; especially when you consider you are limited to 34-pin cards when many expresscard products are the larger 54-pin version.
I think Apple just missed the mark with expresscard. I think it was supposed to be this huge revolution for laptops after PCMCIA slots, but there just haven't been many useful products made in the expresscard format other than card readers and 3G data cards. I probably would have an expresscard card reader, but the 34-pin is too small to accommodate CF cards, which is what I need a reader for mostly.
