Dallas was better! The venue was staged better and the Dome experience felt more grand. It was also more convenient to get to Dallas. Getting to Louisville means that plane tickets are more expensive and you have to connect though another airport with a long layover (often DFW). You get a nonstop flight for Dallas. It’s also probably easier for VEX to bring things to Dallas since it’s close to the VEX HQ in Greenville, TX. This probably lowers registration costs for Worlds.
Edit: Dallas also has two airports, DAL and DFW, so you get a wider variety on arrival and departure times and ticket prices. Louisville airport is smaller. It’s also easier for international teams to get to Dallas.