Likely because Brinstar Depths was hated with a passion by a good amount of the vocal community. Even without Depths being added again, there's so much they could have done with so many of the Prime areas...yet it all goes to waste.
I think the more likely reason Depths hasn't been brought back since Melee is that Brawl's and Smash 4's character models are bigger than Melee's, which makes an already hard-to-traverse stage even messier. If they really gave credence to fan outcry regarding stages, we wouldn't have 75m.