He didn't commit to doing it full time for Legends, so they put him on a bus to Earth n, and recast ed the mantel with the other firestorm. From Bleeding Cool.
BC: In regards to the real life timeline how close together was it finding out that we were going to see the death of Firestorm on The Flash to you getting the job with the X-Files? Were they kind of overlapping? How quickly did they occur?
RA: They were completely non-related. The Flash was always a non-contract role so it was always just whenever I was available I would come in and shoot. The reason they had to kill me was because I couldn’t be a part of Legends of Tomorrow and they needed to have Firestorm be one of the lead characters so they had to kill me to introduce a new Firestorm and then Berlanti, because he’s so amazing, told me they would bring me back as Earth 2 Firestorm/Deathstorm. But I actually booked X-Files right after shooting a movie in Montreal with Kevin Spacey called Nine Lives. I wrapped on a Wednesday, went home Thursday, got the audition Friday, went in Saturday, found out I got it on Monday and found out I was heading back to Vancouver so the nice thing was I got to go and hang out with some of The Flashcast and hang out a little bit.
He also makes fun of the whole "not really dead dead" and parallel universes. So Ronnie firestorm will be back. Eventually.