Why do some books start out numbered by Roman numerals and then switch to regular numbers ?

Answer 1

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Actually, that's an interesting question. Something I haven't thought of before. 

My guess is that it is written separately from the actual book (like a forward, or similar style of opening) from a guest author. So the page numbers aren't officially part of the book, thus the roman numerals are a sort of signifier.

Answer 2

Like your best answer said, the Roman numerals are the preface, not the actual book material.  In some textbooks, the preface can take up more than 25% of the pages before the content begins.