the travel to/from Europe would make it difficult for players to perform well. there's a significant time difference. besides, the NBA couldn't make it in Vancouver for crying out loud. There's also major US cities that have no teams that are certainly large enough to support them: San Diego, Seattle, St Louis, Pittsburgh, San Francisco (Golden State doesn't count because it's in Oakland), Tampa, Kansas City, Buffalo, Baltimore (hey, other sports can support a team in Washington AND Baltimore).
Get the NBA to show it can put a quality team in some of those towns and perhaps get into Montreal or support their franchise in Toronto and maybe they can realistically consider a separate set of teams in Europe.
I know, Europe supports a lot of teams but those teams are already established with their own sets of stars and methods for staffing those teams. NBA would have a helluva a time building a separate league there without trying to steal talent from the other leagues or watering down their own product in the states by taking talent from there.