In general and in this instance, there's rarely a single motivating factor.
I have no doubt members are motivated by the seriousness of Jan 6, the threat to democracy, and the critical need to shine light on the facts.
I also have no doubt there are partisan motivations, based on human nature. Death, taxes, and partisanship.
To varying extents, there are partisan motivations behind every congressional investigation that has ever occurred (Benghazi being a freakishly extreme example). When you dismiss this investigation because of partisanship, you may as well dismiss the concept of congress conducting investigations.
And when you consider that Republicans rejected non-partisan and bi-partisan investigations, and also consider Cheney's prominent role, your objections amount to partisan whining.