I believe capacity was a big factor. If social distancing had not been an issue, that might have made Dickies a better choice, especially since the layout is better suited for a rodeo than a ballpark.
Dickies Arena has sufficient capacity to satisfy their home tenant: the FWSS&R. It was built with that in mind. Even if it had a capacity to match the AAC in Dallas, it would still miss out on some events due to the size. There is a market for facilities of its size and it was well on its way to being of much better use than its predecessor at the WR complex and the convention center arena. The only thing holding it back right now is the pandemic and the economic effects of that.
Edit: I re-read you question and felt like I didn't understand it the first time. I believe they relocated from Vegas to this area because there were fewer restrictions regarding public events in this area. I don't know that the arena capacity in Las Vegas had anything to do with the decision.