Talent, that's the first thing I want to say, to be able to make so much in 1 year must take a lot of talent and dedication, and to make fun out of so much must take even more talent.
Look at other games like Half-Life 2, that game took Valve 6 years to make and it is a lot smaller than San Andreas, Valve might have some laid back lazy developers who take their time. I think Rockstar and Valve are about the same size of company and even though Valve made a new engine for Half-Life 2, you would expect it to take less than 6 years to be made, same is true for Doom 3.
Grand Theft Auto San Andreas is probably the biggest game ever made.
After Vice City and Liberty city you'd think they might run out of passion, ideas, or powers. I mean don't they get tired of making such a huge game every 2 years, Valve and id Software will probably not release a new game for the next 6 years and Rockstar releases such a big game every 2 years.
Apart from it being insanely great game and a lot of fun, this game is HUGE, I can't think of any game bigger than this when I write these lines, you have 3 big cities, connected by big country side and deserts with a lot of nice areas to find in the desert and the country side.
Other games of 2004 and 2005 like Half-Life 2, Doom 3, Far Cry Battlefield 2, are all big games, but none of them is as big is San Andreas.
And none of them allow you the amount of gameplay opportunities as San Andreas allows you to play, you can be everything you want, a cop, a medic, a firefighter, taxi driver, thief, pilot, pimp, train driver, construction worker, gambler, gangster, lover, valet. You can also change your hair, put tattoos, change your clothes, eat, exercise, and much more.
I salut to Rockstar for having such a great talent, skill, dedication, and nonstop art passion and motivation.