I would imagine it's because the 3.5 12 was a modification of the existing 12ga. 2 3/4- 3 in. platform to do double duty as a 2 3/4 field gun and a 3.5 duck slammer. There's no way to do both perfectly so the manufacturers have made the assumption that people want a versatile gun at the expense of recoil. the 10 ga. was built from the ground up as a 3.5 in. duck/goose gun and weighs accordingly.
"Suppose you were an idiot, and suppose you were a member of Congress, but I repeat myself." Mark Twain