Depends on what kind of tie you're wearing, and whether you're shooting video or not :=)
Seriously, probably the main reason is that it's easier to use the trap as a load bearing wall if studs are vertical - then it's just a follow-up to put the slats across the studs. Acoustically, there wouldn't be much difference especially for lower frequency traps - because the wavelength of the affected frequency is large with respect to the trap size... Steve