He got shot right in the "heart"
I love Brawn but he is dead. If not for his whole back torso exploding from the shot from a gun mode Megatron, then the shuttle going up in flames and crashing when it reached Autobot City.
Any "cameo" he makes in Season 3 is due to AKOM being a goddamn mess with characters (one moment he is fighting on the Decepticon side)
The only argument I can give (and it's REACHING) is his tech spec
Outside of Optimus Prime himself. the only Autobots with an ENDURANCE of 9 is Ironhide and Brawn from the 1984.
Ironhide took multiple shots to the chest (Starscream's null rays, Constructicon blasters and finally Starscream switches over back to Megatron in gun mode to blast some more).
After all that, he was still somewhat moving. One would assume Brawn with that ONE blast to the chest would be able to handle it.
Also Brawn was shot by Megatron during the show and got up from it.
https://www.youtube.com/watch?v=NIK0aasdeO0 (Jump to 52 seconds)
Anyhow the real answer?
Hasbro needed some toys dead for new toys.
I love Brawn but "He's dead jim". It's called Warf Effect in fiction.