By saddling the writer with tons of syntax and gotchas that is hard to keep at the top of his mind when trying to create.
Languages like c++ and java might be powerful, but there are so many hurdles that are between start and end that unless you are proficient in the language or have a desire to learn the language, it's very tempting to just give up.
Languages like ruby and python are not fast, but their syntax is so straight forward, the effort to go from a to z is a fraction of other languages, leading to the developer to be able to deliver faster.
From the point of view of a developer that loves to learn, simpler languages inspire me to learn more complex languages, which in turn gives me more opportunity to enjoy my chosen craft.