IBM wanted a homogenous solution that played to its expertise in multiprocessor coherent systems, one which we see in hindsight won decisively.
In retrospect, the SPE was what you design if you have limited experience with complex memory pipelines and processor cores, and an lack of experience in self-scheduling hardware linking them together. It was then topped off with some strange internal politics and Toshiba's desire for a split-off media processor it could barely find a use for. The harsh lesson in relying on exotica and backwards tools seems to fit with the acceptance at the time for cheap, primitive consumer electronics with limited functionality and closed, proprietary software tools that try to band-aid over glass jaws.