After years of incremental develoment, the early to mid-'90s saw the rise of the first Massively Multiplayer Online Role-Playing Games. Arguably, it started with Neverwinter Nights in 1991, which brought Dungeons and Dragons to life and allowed users to wander around a fully-realized graphical world. This early Neverwinter Nights iteration only lasted five years, but its many users (up to 150,000, all willing to shell out six dollars per hour to play) proved there was a significant market for MMORPGs. For better or for worse, this opened the door for later endeavors like Ultima Online and Everquest to start charging monthly subscription fees to offset the high cost of development (and to be fair, these fees were a much better deal than charging gamers by the hour).
These games, too, were massively successful, opening the floodgates for more MMORPGs like Dark Ages of Camelot, Final Fantasy XI, and the sequel to Everquest. World of Warcraft arrived in 2004, hooking fans with an insanely-popular MMORPG that brings to life the world of humans, orcs, and undead first glimpsed in games like Warcraft II and Warcraft III. WoW managed to net millions of subscribers, and while the overall number of subscribers has declined in recent years, it remains the only MMORPG that has millions of subscribers while still preserving its monthly fee instead of going free-to-play.
With World of Warcraft still going strong nearly a decade and a half after it debuted, developers are still scratching their heads and trying to come up with a way to copy what makes WoW so addictive and successful—and no one's truly come close.