You're actually a mana short to BW->U-bounce (costs 1RUU (GP+BW+Bounce)). Even Pulverize wouldn't really help (assuming you had another Volc) due to the contention with Rain of Filth.

I guess a better question would be why Lab Man is in the deck against UR Delver, the only deck in the format playing 10ish ways of killing lab man plus hasty threats and pyrostatic pillars/eidolons to get you if you pass the turn.