Why are spoken languages more common than signed languages?

There's a lot of types of signed languages (some more like sign vocabularies), so not all have the same properties. Also, they are not limited to deaf communities. Most famously, the Plains Indian Sign Language was used both within and across cultures. It certainly wasn't just a sign encoding of spoken language which is what many non-deaf sign languages are.

However, there are no instances where signed languages developed prior to spoken languages. Nor are there non-deaf communities that only use a sign language as opposed to spoken languages.

Again, why questions in linguistics are not a safe place to go but if I were to venture a guess, I would be very skeptical about any explanations relying solely on communicative utility. You can make the argument that in many situations (combat, hunting) signed language would be much more appropriate.

I would suggest that a better candidate for an explanation would be a simple co-evolution. Symbolic communication was simply associated with vocalizations from the start. Since the origins of language have as much to do with socialization and expression of emotion as communication, I would imagine that mapping symbolic meanings on vocal expressions is more productive than with gestures (although paradoxically, now, gestures are used mostly to supply affective content - although in a much more subtle way than just emphatic waving around of hands). So any grammaticalization would happen on the vocal part of language first. But it's a completely speculative guess (although one that would probably have been endorsed by Jespersen).