I'm sure there's some degree of just not being used to multi-threaded code, but it's also not like today's multi-threading. When we think about utilizing multiple cores today, we're thinking about running normal application code across many identical cores, within one process managed by a multi threaded kernel.

The PS3 wasn't like that. It was a single core machine, with one big CPU core which runs the operating system and your application, and it had a bunch of co-processors. I've never written for such a system, but I'd assume that a somewhat close analogy would be a single core Linux computer with 10 microcontrollers attached. (Though I'd love to hear from someone with more knowledge whether this is a reasonable analogy.)