A couple of reasons:
Swift Swim pokes get their Water STAB boosted by rain, while Chlorophyll pokes don't.
Sun boosts Fire type moves, which Chlorophyll pokes are weak to, while Rain boosts Water type moves, which Swift Swim pokes resist.
There's probably another reason or two in there but I can't think of any other.
Swift Swim pokes get their Water STAB boosted by rain, while Chlorophyll pokes don't.
Sun boosts Fire type moves, which Chlorophyll pokes are weak to, while Rain boosts Water type moves, which Swift Swim pokes resist.
There's probably another reason or two in there but I can't think of any other.