I think it is because Chlorophyll sweepers don't get a power up for their STAB moves in the Sun like Swift Swim sweepers do for Rain. Also, the Fire types that get boosted in the Sun are usually Stealth Rock weak, and they are all somewhat vulnerable if the Sun isn't up. While that is also true of Rain, their typing also has a somewhat decent synergy (like the core of Mantine, Ludicolo and Seismitoad). I used a Sun team during the Underdog challenge, and I actually liked it a lot. Priority Sun from the likes of Illumise was crucial in the team's success since I was using Victreebel and Emboar as the main sweepers. The main problem with Sun teams is the difficulty in creating good type synergy. You already have Fire and Grass types, so a Water type seems perfect to create a solid defensive core, but very few Water types want to be in the Sun (and the defensive ones slow down the momentum/offensive pressure). I actually like using Cloud Nine Golduck as my Water type simply because Sun doesn't affect it at all, so it can accomplish some things that you would expect a Water type to accomplish (i.e.- take hits, set up, tank Fire attacks, etc.).