The phenomenon in math and science is to dumb down the content and add more pictures. Compare a chemistry text of 20 years ago with an 'updated' one and you'll be shocked. The topics are treated with less depth and detail today and somehow schools can't 'cover' as many of them in a school year. I know of one district in MD that offers two years of chemistry to deal with the topics that used to be covered in one.
Biology texts may be the exception. That field changes very rapidly and the trend in the texts is toward molecular biology and away from phylogeny. These days, you almost need to take chemistry first in order to better understand the biology texts.
Not to mention the money funneled into publishing firms which, I'm sure you have noticed, are churning out the LIBERAL point of view every chance they get.