i think it's just simply coz at that time when they started there was only microdrives and then CFs which share the same drives... then it had better appeal to the pros who already have such cards... i believe this is partially y canon has not made the switch.. people will have to spend money to change card formats.. CF cards are also bigger, so less easy to lose and are generally tougher than the smaller thinner SD cards (i think)..
SD came later... but the smaller size and better value will appeal to many new users, although it is slightly slower, but only by one step (Sandisk just announced Extreme III SD cards).. capacity wise, it's the same.. there are 8GB SD cards around now...
this could be the reason behind the switch... it is a pulling factor if u compare the costs of CF vs SD... and the better portability and compatability with hundreds of other cameras/handphones/pda...