They want to make Traps and Gears different, that's why they change the symbols.
Makes alot of sense because you can use 1 of each or 2 of each now in the game and by changing the symbols, there is a clear black line for the little tykes or parents of said tykes to understand the difference. Again though, it appears they INITIALLY planned to match the gear attribute and mold color to the Bakugan attribute and mold color BUT later decided to keep them separate and/or game designers thought up the Gate card color gameplay element.