Because you are not on an immediate timer when using the specs set. If you wanna use the guts facade set, you are on an immediate timer due to the flame or toxic orb and the enemy can just spam a recovery move and that's not counting more residual damage in the form of stealth rocks and rocky helmet. With the specs set, you are not on an immediate timer and boomburst's base damage makes up for the rather pathetic low base special attack. In gen seven though, Swelllow did get buffed in the form of twenty five more special attack and further solidifying the specs set
Source: https://www.youtube.com/watch?v=wjkrt-J2TBw
