... Why doesn't the average Trump voter expand their hand to understand the other side?
Because the price of owning up to historical and contemporary fact is way, way,
waaaay too high, and impacts too severely on self-image, especially for those whose main claim to fame is to be from where they were born. Same goes for most any other people or nation. So:
myth.
This dire need to protect self, along with the sad fact that commitment to democracy has ever been wallet-deep, guarantees the rise of Trumps when reality contradicts the promises of national/tribal myth, or it seems threatened (Obama). Which is why you have to keep such myth in check, meaning why you need to keep today's GOP in check, while merely fretting about nonsense on today's left. Both sides, however, share the same addiction to Rosy Mirror, just with differing predilections about what they want to see, each meanwhile listening to their own preferred peanut galleries.
Luckily, in the world there are skeptics, who have all learned to reason from first principle. I mean,
none of us fall prey to such transparently artificial contrivances as nationalism and patriotism, right? Surely.