FFS.

For the millionth time ..... 

MGO is ERC223.

Bittrex infrastructure is not ERC223 compliant.

If/when it is, then they'll list MGO.

When than will be, nobody knows. You'd need to ask Bittrex.

Is it in their (Bittrex) interests to upgrade their infrastructure? Yes. Given that Ethereums development team seem to be behind the ERC223 standard.