I'm curious to know the real answer.
I do know that you are not allow to make a market in your own stock, so IB cannot act as a MM in its own shares. Does offering short capability fall under this restriction? (Or perhaps to avoid falling into this restriction, you need to pass the short off to someone else and IB just didn't bother to build that infra?)
Also, I recall someone made a snide remark about how quickly IBKR options were available -- hmm..... I think BX was even faster.