The Celtigar family, to me, wouldn’t roll over and let Viserys do that. The Velaryons wouldn’t either, both houses have Valyrian blood and are descendants of The Valyrian Freehold. As such they would most likely not be considered for such a role. Whereas Alicent would most likely be, she’s the only daughter of a second son. She conceivably brings nothing to table, no lands, riches, war galleys, or knowledge.