Why is Airdrop between MacBook Pro (Yosemiti) and iPad2 not possible?

Appler has not said why. All they said is the the iOS device has to have a Lightning connector.

Use AirDrop to send files from your Mac to another nearby Mac or iOS device - Apple Support

The iPad 2 does not have the hardware to support it I assume and I do not know what is limiting

As I said, Apple has not said why