He never won
That explains why Trunks' timeline is gone and the people Trunks fought to protect have been erased along with Zamasu. That explains why Goku had to resort to calling the Omni-King to even have the hope to put an end to Infinite Zamasu.
He got sliced in half by Trunks
Who was empowered by all life on Earth and two Saiyans, plus Zamasu's immortality had been greatly weakened by Vegito and Goku (Gowasu himself states that was his only weakness, and they exploited the unstable balance between Zamasu's immortal soul and half-mortal body to succeed).
And actually, if you paid attention to ep. 67, you'll know Zamasu willingly casted down his form as a God to merge with the Universe, so Trunks didn't destroy him. That was impossible, half of Zamasu's body was still immortal.
and erased by Zeno
Along with the entire Future Multiverse and all mortal life in it.
Combine that with the fact he got lucky Vegito defused while he was getting his was handed to him.
He survived the Final Kamehameha and his body somehow regenerated even though Vegito's final punch had critically damaged Zamasu's face. Vegito himself changed his expression from his usual cocky one to a more desperate and frustrated one (even combining all his power into one attack to end Zamasu quickly ----> that's not Vegito's usual method).
I suggest we stop here, since it's off-topic. And yes, i was obviously talking about Fused/Merged/Infinite Zamasu. Call him however you want, they are the same being.
At his core Zamasu is good like Shin, though I guess you could say he was so fastidious that it backfired. But you know, for this "Future Trunks Arc" you had to depict Zamasu and Trunks' inner conflict, right? If this was back when I was drawing the manga myself then I doubt if I could have done it. I mean, I'm not very good at depicting the characters' psychology on the page. So this all came together because now I only have to think up the story. [...] On my own, I doubt I would have been able to express Zamasu's fall to the dark side.
Akira Toriyama, DBS vol.4 joint interview with Toyotaro