I'd like to submit that it's because they are obsessed with those things themselves. There's something appealing and romantic there, and there's the added bonus that you can hide behind the charge of "prejudice" whenever the object of your fascination is criticized.
To be more precise about it, there's a hyper-vigilance for any hint of negative bias - but no awareness of the possibility of positive bias. The leftists will not admit to being enthralled, smitten, infatuated with those very same characteristics.
Obviously, that is itself prejudice, but they'll never admit it. To acknowledge positive bias toward "marginal" or "minority" groups would call into question the basic assumption of entitlement liberalism - the assumption that white Christian prejudice is so universal that it can only be countered by a compensatory bias.
It is obviously true that there still exists in America today, an element of the old, pernicious white racism and Christian supremacism. But I think it's also true that these things are in decline, and that there are other forms of discrimination that the leftists aren't so fond of talking about.
To acknowledge that at some times, in some places, there might be a positive benefit to NOT being white or Christian would endanger the system. So, for example, Elizabeth Warren's Cherokee ancestry may not be questioned.
But back to Obama. Where did people get the idea that Obama was born in Kenya?
Barack Obama, the first African-American president of the Harvard Law Review, was born in Kenya and raised in Indonesia and Hawaii. ...Whether or not Obama personally wrote or edited the text of his bio in the 36-page pamphlet, he allowed the words to represent him.
Now, where do people get the idea that Obama is a Muslim?