Because the Republican party decided that because Trump was no longer president at the time of his 2nd impeachment trial he couldn't TECHNICALLY be convicted:

https://www.msn.com/en-us/news/politics/fact-check-yes-mccon...

Which, if you paid attention to the trial was extremely well reported, so either you genuinely didn't pay attention at all or you're trying to imply anyone in Congress seriously believed he wasn't culpable.

So to answer your question simply, he wasn't convicted because it wasn't politically expedient to the rest of his political party.