What I would guess:
Maybe most buyers considered the bumper to be less useful?
It adds to the size of the phone in all 3 dimensions, collects and is not exactly stylish.
Without the bumper the phone has a much more elegant appearance and is looking more like any other phone; which many might prefer.
And maybe it was not sold as an accessory (different colors were another demand, if my memory serves me right), because a) demand was not heavy and b) it could not be made from recyled materials, due to the necessary flexibility.
The FP3 housing did not contain any post consumer recyled materials, while the FP3+ housing contains 50%. My bet would be, that the bumper would have reduced that percentage.
And offering the bumper as an extra accessory would depend on demand. Most likely this would be limited besides the protective cover. And the less they can sell, the higher the cost for production. The higher the price, the lower the sales etc. etc.
But since this is just guesswork, maybe it was just, that Fairphone themselves did not like the bumper.