Honestly, you might have had a better time of that if you'd participated in the informal vote of which victory to aim for on D1, rather than trying to lie to everybody and go for that ending all by yourself. The fact that cultists can 'defect' to open-source ending means that you could equally try to convince town to defect to closed-source ending. If the majority of people agreed that cult victory was desirable, then that's what we would have gone for.
Instead, your attempt to go for cult victory secretly meant that cult victory had no public support and was doomed. You could have changed that if you'd argued for it! You wouldn't even have to admit to being cult to do so, just claim that you're a town who wants cult victory or something if you'd prefer to keep the secret.
^Pretty much this. There's a bit of an issue here that you were sorta on your own, and not only that, but it was already far too late to be able to WIN as the Cult. The other unfortunate fact is that there were less Cult in general, making it harder to accomplish.
What incentive would staying Cult be to me? I'm not even sure what incentive COULD be made to stop me from trying to go for a Town win. That's the tough part that I see. I'd say that adding in a partner to the SM might make it better.
Alright, here's my idea that might make it harder to do:
Make an SM-ally role that is functionally the same as the Cult. Make it so if they're voted, the game is over for Town. Same for the other direction (Town SM-Ally). Maybe this means that Cult loses power by converting (Since the Cult can essentially be taken over by this rogue agent.)
Personally, a Cult SM-Ally sounds the most disastrous for the Town, but it'd be much better for the SM to be able to use any power in the game (Just one Action per Day, though). This lets the SM hide as any role, and allows the SM and the SM-Ally to functionally lockdown the Town if everyone claims at the wrong time.