The real reason - The Surface is a real, full function PC capable of being a business-ready machine. The iPad is a media consumption device.
I agree with this. The only way my iPad Air (company owned) or my iPad 2 is remotely usable as a serious business-capable device is to have both paired with Bluetooth keyboards/keyboard cases. Even then, without remote desktop capabilities with the office, the devices revert to their native state, media consumption.