It's because you don't have an argument.
You know he is right but you don't care because you see the Religious Right as your "useful idiot" rubes to vote for your policies.
But you felt compelled to react defensively in some way, which is why the eyerolls are here instead of just ghosting the thread.
I'll elaborate for you.
To someone who is a conservative/traditional/orthodox (select your favorite adjective) Christian, Buttigieg's comments come across as elitist disrespectful posturing. His argument boils down to "Trump's so narcissistic and focused on his personal wealth and power so he can't possibly be a Christian." It'd be easy to make the same argument about Buttigieg himself, i.e., "He's living in a depraved charade of marriage and supports the legal killing of the unborn so he can't possibly be a Christian." (Please note I'm not actually making that argument.) The principle is the same, it makes the group that already agrees with you feel better about their beliefs by coloring them as "true Christians," and justifying their disapproval of their opponents. At the same time, it deliberately offends those who disagree by castigating them as unchristian, basically implying they are charlatans.
An additional factor is similar to why it's annoying when atheists make arguments against Christianity based only on non-observance of Levitical law: it misses the entire point and reveals a total misunderstanding/disregard for our actual beliefs. Or for an example which may be easier to understand, it's like telling a Jew who supports the rights of Palestinians that that belief makes him not Jewish and possibly even a neonazi. By saying that Trump and right-wing Christians don't care about the poor, Buttigieg is denying the reality that we actually do and actually merely disagree with him on how to best help the poor.
