Definitely true that maths and sciences have way more cache than arts subjects now. In "my day" the clever kids took English, history and a language. We had three English A levels classes at my sixth form, two history and only one for maths and each of the sciences.
At ds's slightly smaller, independent (but very 'all-round', arty/musical) school, there are only going to be two classes next year for maths, physics and chemistry. The rest are all one class.
I think it's mostly parental pressure. At the high-performing state school I work in, parents of clever kids won't let them consider double science, or art or music GCSE, or English A level. They have to take all maths and science subjects. Some are even anti reading for pleasure...