Not impossible for all wii u games, but again.
How would you put a disc in a portable device.
That would make no sense to do, and thus physical copies would be a no go and it wouldn't be fair to WiiU users if they gave free digital copies of physical games for WiiU ported to Switch. Plus there would be no good way to prove your ownership of the game without it being easy to trick the system (if they had some way to read a disc and then give you a digital game people could just share the disc around). They probably will re-release WiiU games for switch, but you have to realize not all games supported playing only from the gamepad.
If you look at it from any logical standpoint there really would be no good way to have free backwards compatibility.
Edit: It would be possible for eShop purchases, but any physical copy would not work in any good way.