Nobody has direct evidence of the shooter's motivations or political affiliation today. Nobody.

That could change, but today this is true.

Any evidence we have is circumstantial. Most of that circumstantial evidence points in the direction of an extreme right winger.

I'm sure there's some circumstantial evidence pointing in the opposite direction, but it pales in comparison to the circumstantial evidence pointing in the far right direction.

Much of the circumstantial evidence pointing away from the far right came from a mis-attribution from the FBI to a trans rights marker that was debunked as categorically untrue. Or statements from the Utah Gov which are (AFAIK) not backed up by any actual evidence other than how he "wishes" it was. Or from Trump who is known for lying and making things up.

I'd be curious what evidence they have that hasn't been announced/released yet that makes them think that.

I suspect they want it to be true that he was left leaning and they may be assigning too great a weight on the debunked FBI claims, the Utah Gov claims, and Trump's claims; despite most of the circumstantial evidence that's been released so far pointing in the direction of him being far right.

To be clear, this could change later today if the investigators were to release evidence about the shooter's motivations. Until then, all we've got to go on is what circumstantial stuff has been released so far, and that's largely pointing in the direction of the far right.