There is a certain group of people (counting me too) that are used to working with strongly typed languages. It's really hard for that group of people to understand how technologies like php, html, javascript (..., rss, perl, ...) become so popular.
The answer is (in my humble opinion) the ease of use and rapidness. Everybody can learn to use them. Sacrificing the good style and the heavy approach (that comes from the heavy books) comes out be (in most cases) an acceptable loss.