A guy who really enjoys history is likely to be thrilled by the prospect of an in-depth class on the political environment of the Italian Renaissance. On the other hand, there's people who couldn't care less about such a subject.
There are people who enjoy Trig or who will use it in their expected careers. Then there are others who simply loathe it and will never use it in their life.
The idealists and supporters of the US school system believe that this current way exposes everyone to everything and so everyone can be equally good. But really what ends up happening is that everything gets dumbed down to the point where everyone is equally bad.
There are very few students who can be Renaissance Men. There are very few people who have expertise in all the traditional areas of schooling, few are good at math and science and history and English and art. On the other hand, there are many students who excel in one or two of those areas and so it makes sense for those who are really good and really enjoy art to devote the vast majority of their studies in middle and high school to art. There are those who are really good at math, it makes sense for them to devote the majority of their studies to mathematics. In doing so, we breed better artists and mathematicians rather than starving the artists and mathematicians in courses that they will never fully master and bringing down those who are good at that subject.
The US education system apparently has not come to the realization that ability differs.
To use a sports analogy, its a bit like taking Apolo Ohno and Peyton Manning and telling them to throw a football 20 yards. Ohno is unlikely to ever need that skill (being an ice skater) and indeed not being a football player he might not even have the ability to do that. On the other hand, Manning isn't really challenged by this and is unlikely to really improve (because there's no support for learning to throw the ball any higher). Both Ohno and Manning are both brought down by this, Ohno because he has no motivation and little ability, and Manning because it is too basic. Instead, Ohno should be improving his speed-skating skills and Manning should be improving his passing skills (beyond just 20 yards!).