Great question. The simpler syntax reduces the number of rules to remember and shrinks the possible space for bugs. In back of the envelop user testing with non-programmers we've found people can pick up Space significantly faster than JSON.

JSON is really popular. However, the number of people in the world who can read and write JSON is say, between 2-20 million. So a very small percentage of the people who could benefit from reading and writing structured data are currently doing so. Our thought with this is that perhaps if we can get enough adoption and developers involved over the next year or two we could enable a lot more people to collaborate on the web.