Because the odds were not in the rebellions favor.

So as Darth Bane declared, there can only be two sith, right? Well, the reason the rebellion won is not because of hope, but because of the force. The force had betrayed the empire because as we saw with Disney there were multiple sith. In this sense, the force betrayed the empire, and thus the rebellion won.

Well Bane wasn’t referring to the Sith species, which is what we see on exagol in Rise of Skywalker. Also Bane didn’t write that rule into the force, he made it so the Sith could live in hiding. Besides there are other examples of more than two Sith existing at once: Darth Plagueis’ master had two apprentices at the same time for example.

we attack a space fortress and lose all our attack force and the last jedi in the empire wil go