It would have fit in the show, but: (a) it would have been too convenient and (b) it would have made things so much harder for Elizabeth.

Would she have been able to go through with this plan, knowing that Young-Hee has been through so much thanks to Russia?

And how would Don respond? We still don't know how the Russians are planning to use this against him, but having his mother killed by the Russians might actually make him more resistant to pressure.