My guess is because you either aren't reading enough of the explanations OR you need to do some background reading. I found the background reading extremely helpful on this test, to kind of put all the pieces together. IC was a great resource but I needed some sort of framework to put all the IC facts into.

If you have time to order the REA guide to Western Civ. I, do. The practice tests in it are very helpful and have detailed answers.

Also, try this SparkNotes link: History Study Guides - SparkNotes Click on any one of these topics and it'll give you overviews and explanations and also a review test.