Why are retro classics like the early Ultima games popular? Is it just nostalgia - adults pining for what they enjoyed in childhood/adolescence? Is it because mobile phones, PDAs and mp3 players offer a similar a similar gaming environment to the Apple ][, Commodore 64 and TRS-80? Could it be that these some of early games compensated for their technological limits by having better gameplay? Or has this renaissance been caused by the strengthening of the open source movement?
If you're curious, try Lairware's remake of Ultima III (this particular version is for the Mac only). It's shareware. Registering it for $15 will let you finish the game, but you can waste days on this game before you get to that point. It's very polished.
There's also xu4, a freeware version of Ultima IV for Linux, Mac OS X and Windows and other platforms. Ultima IV is a bigger and better game than Ultima III, one of the best games of all time, imho. xu4 is not yet playable to the end, but they're getting there. The personality quiz at the beginning of the game is reason enough to give it a try.
There are a lot more resources on retro-gaming, even other new versions of these games. Not to mention all the sites about running emulators of old operating systems on modern ones.