I think there's a cost vs benefit thing. The tax credit on bus passes directly targets public transit users. Every single person you hit will be a regular user of public transit, simply because it does not make sense to buy an $80 bus pass for a $12 tax credit unless you receive $68 or more worth of transportation in return.

With a bike credit, if its goal is transportation, there's going to be a LOT of misses. How many people do you think buy a bike every year and never, ever use it for a transportation purpose? I'm guessing it's a pretty large portion of bike purchases. And indeed the tax credit would disproportionately benefit racers who buy more expensive gear (a cap could help with that), but probably won't be using that gear for regular transportation.

If the goal is to make the country more active, as a gym-membership credit would be, then there'd be fewer misses. But still lots...the purchase of sporting equipment doesn't inherently make you more active...a lot of it is bought with good intentions and then sits in the basement. Memberships, while many are bought impulsively, get cancelled if they're not used.

Bike stuff is expensive, but if the savings over driving, plus the health benefits, aren't convincing you...will a 15% discount really make the difference?