???
You got me. I wonder why are some people are better at spelling and punctuation than others.
Because some people have to be the spelling masters.
Dammit! Beat me to it. 
But most can spell “good” and “aren’t”. 
I think Derek Zoolander needs to start up a new academy - skool for kids what don’t spell godd.
And someday we will need a - ‘place for kids have godd gramer learned at them’
Anyway, I am dog-doo-stinkingly wank at basic maths (arithmetic, especially multiplication and division) but I am average at more complex aspects. And above average at algebra - probably why I became a good computer programmer.
I guess extensively considered answer to your rather complex question is - Because.
I agree with Lobsang that there isn’t really a single answer to this question, but a couple of factors probably contribute to the fact that so many people are terrible at math:
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There are more poor math teachers than, say, poor English teachers. Most people who are really good at English have strong communication skills, while plenty of math experts may understand a concept but have trouble explaining it. At the same time, a lot of teachers in the lower grades are math-phobic themselves and therefore have trouble teaching effectively, while relatively few hate reading or history.
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Math has a well-established culture of failure. Children (especially girls) grow up surrounded by well-meaning adults who say things like “Ooh, math is hard, I hated it when I was at school and I can’t even balance a checkbook now.” The same people would never dream of telling a child that reading is a miserable experience and plenty of adults get along without it.
Heard a rumor once that families that tend to be mathematical throw off the odd musician and that musical families will spawn the odd mathemetician.
Does anyone know if this is duff or not?