Grammar Nerd Wednesday: Why is Poor Grammar a Subject of Ridicule?

This is part of an ongoing series of posts designed to make everyone think I'm a colossal prick because of my grammatical specificity. These posts are either me lecturing the masses about how to properly use grammar/punctuation/the rules of the English language, or me figuring out for myself, textually, the aforementioned.  They will run every Wednesday.  If you run afoul of these rules, rest assured, even though I judge you for your poor grammar, I'm still a lesser being than you.

            When you think about it, poor grammar is a failure of sorts.  When someone is bad at mathematics, no one stands around laughing at them for not being able to understand factorials, or thinking “Jesus, that guy is dumb!” But, when my Boss’s boss sends around emails with no concept of the correct usage of your/you’re or there/their/they’re, I judge.  It’s not fair, but I judge.  Hell, these posts are all about judging those who exercise poor grammatical usage.  Why is it so much more open to ridicule/a bellwether of intelligence in a person?