Why do they only use two of the three sections SAT test for your score?

The SAT doesn't use only two of the three sections for your score; it uses all three. There are three scored sections: Reading, Writing and Language, and Math (with and without calculator). A fourth section, an unscored experimental section, is sometimes included to test potential future questions. This experimental section is not used in calculating your score. Your final SAT score is a composite of the three scored sections.