The main thing that's changed is the labor involved to make multiplayer work, the technology available to make it work well and the user base of people who would make use of it.
Let me give you one example: Multiplayer in a TBS game only works if it's seamless for people to play a few turns and leave and come back later to the game with their friends. In 2006, that kind of infrastructure would have been extremely expensive. Back then, every server was a physical server. Now, one big powerful server can have hundreds of virtual machines running copies of the "server".
