“why is it so boring to do something encouraging, supportive etc. for Christian Americans?”
Who says it is boring? And if you think the US isn’t “encouraging, supportive, etc” of christian americans you haven’t been paying much attention.
“why is it so much more fascinating (awarding? satisfying?) to find faults in another guy who tries to do something encouraging, supportive etc.?”
No one is finding fault with Barton because he is trying to be “encouraging, supportive.” They are finding fault with him because he is spreading a false and misleading narrative to support is own personal prejudices.

Christians, American or otherwise, should not expect to be ‘supported and encouraged’ by ‘the world’.
Specifically on Barton: (1) in my view his theology is not Christian, and (2) I do not regard apparent historical revisionism as either ‘supportive’ or ‘encouraging’.

Exactly, Ken.
Some of those who oppose these ‘bartonics’ do so BECAUSE they are Christians, and believe that truth (rather than trying to spin some ‘party line’) is important. And in this regard, such people may well share important ‘common ground’ with many outside the Church. So be it – there’s nothing new, or wrong, with Christians working with others on matters of high principle or practical worth. (see, inter alia, Mark 9 : 38 – 40)

Yes, but why is it so boring to do something encouraging, supportive etc. for Christian Americans? And why is it so much more fascinating (awarding? satisfying?) to find faults in another guy who tries to do something encouraging, supportive etc.?

Yes, in the 90s.