Taking ideas seriously does not fit with the rhetorical style of textbooks, which presents events so as to make them seem foreordained along a line of constant progress. Including ideas would make history contingent: things could go either way, and have on occasion. The 'right' people, armed with the 'right' ideas, have not always won. When they didn't, the authors would be in the embarrassing position of having to disapprove of an outcome in the past. Including ideas would introduce uncertainty. This is not textbook style.