I suspect they decided there was already too much going on in Season 2 to reintroduce the Romulans. They're saying them for STPicard and the Sec31 show, my guess.
One thing that I don't understand is how they'll manage to take Georgiou away from Michael's future and back into S31? That will be an impressive trick to make that believable.
Michael: "Did you see that? Georgiou fell into that temporal wormhole that spontaneously opened before her and closed immediately after she disappeared in it!! I wonder where she'll end up!"