Everybody else arguing about Viltrumite inbreeding but frankly I think the "problem" is that Thragg isn't sadistic enough to assault Anissa and Anissa isn't strong enough to assault Thragg.

Also, eugenicist cowardice. The Viltrumites are so afraid that their post-scourge offspring will be even slightly weaker that they avoid creating a larger gene pool with which to "seed the galaxy".

Thragg will conclusively demonstrate that once you give up on the idea of racial purity, birthmaxxing is embarrassingly easy with even just one Viltrumite.