I'm not MLS historian, but it seems that around the middle of the aughts there was a significant shift: started to bring in older stars from Europe (Beckham in 2007 was one of the first), new teams in 2005 started using European/Latin American naming conventions, started seeing teams build their own stadiums in urban centers as opposed to using stadiums built for other sports or using soccer-specific stadiums in the suburbs.
The target audience in the early years was families with kids who played soccer and has shifted to urban young adults and families who follow international soccer, very roughly speaking