^I'm going to guess Misty is the one you're refering to. Brock still has somewhat of a quest that is shown in some episodes. Misty, on the otherhand, had no definite quest besides being a water pokémon master. Meaning without something water related around, she would go nowhere. Brock's quest is easier because it's more broad than Misty's. To make sure they wouldn't do another Johto mistake, they took out the character with the least potential for the show's sake.
I won't give a definitive answer, but I'll say it's a fine guess.
While it IS true that Brock technically has a quest, he doesn't actually follow up on it. I mean, since he's aiming to be a Pokemon Breeder, why doesn't he, you know, actually BREED a pokemon? I mean, the funny thing is, the only one of the Main Cast who has actually had a pokemon breed and produce an egg is Brock's replacement, Tracey.
Also, as for Misty not having a definitive quest, doesn't Ash
technically not have a definite quest? I mean, the qualifications of a Pokemon Master is unknown, especially in the Anime, and plus, Ash is literally getting nowhere with that Goal (even IF he manages to make champion of the Leagues, it only proves that he will be considered the champion of the leagues, and maybe even champion of the Elite 4. It by no way proves that he's a Pokemon Master [especially considering how Lance, Bruno, and even Cynthia were not labeled as Pokemon Masters.].), I mean, since Ash doesn't REALLY have a definite quest, shouldn't he ALSO have been replaced?
And even IF gaining the Title of Pokemon Master is considered a definite goal, the mere fact that characters with Set Goals (that is, Brock, Ash, May, Max [well, sorta, since learning about Pokemon training early on does technically count as a definite goal] Dawn, and Team Rocket) are being demanded by several people in the fanbase to remove them from the main cast due to their being "stale" (oh... believe me, there are ACTUALLY people stating that Ash should be removed from the main cast due to being "stale and repetitive) seems to imply that a character without a definite goal would be more variable, and theoretically, more interesting than a character with a Set goal, which means that Misty would, in a theoretical sense, be more interesting than Brock, May, Max, Dawn, Team Rocket, and Even ASH.
Anyways, that should be all (I mean, we don't want a flame war, do we?). If you want to discuss it more, come to my talk page, or at least PM me.