As I understand, there is no essential difference between spoken and signed languages. Both have the same kinds of phonetic, morphological, syntactical and semantic complexities, both are prone to ambiguities, both change over time etc. The main difference is one of medium.
Spoken languages have, of course, some advantages over their signed counterparts, as the possibility to communicate without visual contact. But signed languages have, in turn, other advantages over the spoken, like higher "bandwidth", i.e., information is conveyed by multiple channels at the same time (hand shape, hand position, eye gaze and head nodding).
In spite of that, spoken languages are much more common than signed languages. And most of the latter have developed at "artificial" environments, like schools for deaf children. Why don't we see, for example, isolated tribes (with hearing and speaking people) that haven't developed any kind of spoken language, but have a rich tradition of signed languages, instead?