Believe it or not, the underlying reason for developers to make games is to make MONEY! The Sims didn't just make money, it made absolute shitloads of money. So a sequel is just common business sense. Of course, people who "aquire" their games from.......dubious sources often lose sight of this, for obivious reasons.
I'm really suprised by how many here never played The Sims(or claim so anyway), yet seem to hate it with a vengeance. If you never played the damn thing what's your basis for hating it? Because it sold more copies than your two favorite games did put together? Because it doesn't appeal to you? If that's the case, i've got two words for ya: GROW UP!
I played the Sims, thought it was a blast for a week, then never played it again. But if i was a Sims fanatic i'd be pretty damned happy about them regularly releasing expansions with new stuff and features. Hell, i wish more developers would follow in this vein, since way too many games end to soon despite showing great promise.