Why do they not go to COTA? It's a great track.
Of the 36 race dates on the current Sprint Cup calendar, only 4 races are held at tracks not owned by ISC (the France family/NASCAR) or SMI (Bruton Smith). That would be the two races at Dover, and the two races at Pocono.
To add a race at Circuit of the Americas, a current Sprint Cup race would need to be replaced, or the schedule would have to expand. SMI and ISC have zero reason to give up a profitable race date to a track they do not own, as they have dueling monopolies on the current schedule. They will not do it, and NASCAR would never force them to do so (especially since NASCAR is the owner of the ISC tracks).
Dover and Pocono are unique tracks with solid private ownership committed to keeping their tracks, and definitely are not looking to cut their revenue in half by dropping one of their two race dates each year.
The schedule is already too long, but the politics and greed behind the current ownership groups prevent any contraction in the current schedule. Don't expect them to expand it either though, especially to a track not already owned by ISC or SMI. Also, don't expect them to expand it for a road course. Road courses have lower attendance numbers, lower TV ratings, and are generally harder to sell sponsorship and advertisements.