I think it started when, as someone mentioned, many of the "sins" you used to go to sin city for became legal in more places. I also read somewhere they essentially the casinos a while back noted that gambling was on the decline, so they pivoted into being "experience destinations" which is when you saw all the more trendy eateries pop up, and they started doing more musical act residencies, then added a hockey team, then a football team, soon as baseball team, etc.
There's just not much left that's "special" about Vegas, and they thrived when it was this unique thing out in the desert. The only thing propping up the convention/trade show business is that it's the only city in the country that can handle that many out of town visitors in that small a space. (I've seen companies moving company specific trade shows to places like Vancouver, and one even moved it's massive yearly show/event to Austin)