That's a good question.
Part of the trouble I've had with a lot of companies who put out modules and a setting, is they either released some modules, but delayed their setting, or put out a setting book, but delayed putting out any modules or other.
I mean, how are you supposed to run a campaign using them without enough material? Not very well. You can either put it on hold until something new comes out, make up stuff (which will screw you up later), or do what I do, use planar travel, right out of the Elric novels, and move the PCs from world to world by the hands of Fate. Neither is particularly great (though the last is mostly hard on the players)
I mean, Chaosium and DLOM. I know, supposedly it was supposed to be out earlier (like a month after DLOM), but their first module for it was about a year after it came out. (They had also stripped out the intro adventures when they search & replaced Elric! into DLMO)
Sovereign Stone had a somewhat similar problem. They put out the setting book first, then put out a magic book, then put out a race book, then finally put out an adventure a long time after that.
It was somewhat hard to get a feel for Sov. Stone because while the main book gives an overview of the setting, a lot is devoted to rules. It wasn't until I read the first two novels for it, that I was able to get a good idea of how the world felt.
Especially the Orks in it. The Orks in their world have low charisma not because they are ugly, or beastly, but because they are blunt and have absolutely no tact. It wasn't until the novels that this was really obvious.
Of course, it really can't be helped. A lot of d20 companies are very small and have neither the time nor the money to pump out products at first. They have to go slowly, one product at a time. Some, most notably Green Ronin, put out almost monthly updates on Freeport on their web site until the main book finally came out.
But in some cases, like Chaosium, or even Sovereign Press, they had previous, non-d20 editions of the game that they could either refurbish for d20 use, or put up on their website, as background material. A lot of the early Kalamar adventures were like that, I believe.