Why didn’t Viserys I try to conquer Dorne? The realm had been at peace for a while, was prosperous, and united and they had a lot of dragons. You figure some lords would be itching for a war.
Because Viserys I is ultimately a very lazy man, who doesn’t really want to change the status quo. The furthest he’s willing to do is to throw some money at Daemon’s little adventure in the Stepstones, because that helps him deal with his annoying brother, deals with a foreign policy problem, and might get Westeros some additional territory.