The supernova exploded before Spock even met with the Romulan leadership. Putting Red Matter into the unexploded star was never an option.
It was a magic Star Trek FTL explosion, just like Praxis. According to the Star Trek Online
game, the Remans did it by injecting protomatter into the star. Personally, I'm looking forward to the novelverse's version of the event which will be coming up in the next couple of years.