I believe that they didn't use evidence of election fraud or the Capitol event being a false-flag because it would have been enormously counterproductive.

Think about it. If fraud and/or the false-flag would have been made into part of the defense argument, the defense would essentially be backing the rats into a corner. And rats never fight back harder than when they are backed into a corner.

The choice would be A: vote not to convict, and thus tacitly admit that they're guilty of complicity in election fraud and/or a false-flag event, or B: vote to convict in order to keep up the facade of their innocence. You know which way they'd choose.

Thus, making election fraud and/or the false-flag part of the defense argument would have likely guaranteed that President Trump would get convicted.

Instead, the defense team didn't back the rats into a corner, and instead give them the chance to vote not to convict without tacitly admitting to the fraud or the false-flag. This much lowers the chance of conviction.

There are other, far more effective, opportunities to challenge the fraud (possibly before the Supreme Court or some other venue). But the correct strategy for the impeachment trial is "live to fight another day".