What is the motivation behind capitalizing the first letter of each word except prepositions in news, articles and blog post titles?
It's convention? But really, a title is meant to stand out - it is in some sense the most important text on a page - and capital letters help do this. With regards to prepositions, they're of lesser importance within the title itself, and hence don't really need to stand out.
It's really as simple as that, I think.
I agree to Noldorin that the capitalization of titles is convention that titles are meant to stand out. I would like to add that the title of a book, an article, etc. is the “name” of the book, the article, etc. Because content words in a proper name are capitalized in English, this may also be part of the reasons to capitalize titles.