Power would be my guess; even a small spinning platter would be a drain on already-taxed smartphone batteries. If you're talking about putting the drive in the phone, then space would be an even bigger issue.
Anyway, in another year or two, the capacities of micro SDXC chips should be in the 128/256 GB neighborhood, rendering micro HDDs obsolete. I've been using a 64 GB micro SDXC in my SGS2 for about 5 months now with no issues -- just reformat from exFAT to FAT32 and you're ready to go.
