It really is this simple...
If the Republican base were polled and at least half of them said Trump should be convicted, well over the 17 Republican Senators needed would join the Dem majority to convict.
This has nothing to do with office holders principles, opinions or feelings, or even their ability to accept truth and reality. What matters is whether they think they will lose their offices and post-office $ opportunity.
Now, there is a chicken and egg problem with the voters. Do they start out bigoted, stupid, etc., so they are uninterested in factual info and seek out propaganda that tells them what they want to hear? Or have they been subjected to so much propaganda and social pressure from friends, churches, etc., that they truly believe lies?
Clearly the influence of money to perpetuate the manipulation has been facilitated by Citizens United and the current makeup of the USSC has been so skewed that court rebalancing is essential. And our allocation of 2 senators per state despite huge variation in population tilts the scales in Republicans favor. Its hard to change but until it does we have a serious problem.
So, the problem is not that its a mystery why Republicans refuse to look at this evidence and do their duty.