IDE drives were the first ones to popularize integrating the logic controller onto the hard disk itself. You can read more about the logic board and its significance on this page .
Hard disks are heavy, and are mechanical devices.
They just aren't well-suited to being mounted in expansion slots like a video card or modem. The width of the drive would typically block off at least one additional bus slot, meaning one of these devices took two expansion bus slots. Cooling was also sometimes a problem. Of course, all of these are physical implementation issues, and don't detract from the significance of the advance in integrating the controller and disk assembly. It didn't take long until manufacturers realized that there was really no reason to keep the hard disk physically on the controller at all.