They can't.
There's a lot more legal entanglement here than y'all understand.
When Dawn of War 2 retribution came out, they had to re-record dialogue for every single unit. They weren't able to use any voices from the original release or the first expansion. I forget why they had to do it, I believe it had something to do with the game beng a standalone release that was utilizing steamworks rather than GFWL.
There's also the matter of budget. An expansion like Brits will get far more resources allocated to it than commander DLC.
Commanders cost more than the dlc of vcoh lol.