Every engine can run in 60fps, thats not an excuse.
I also dont understand Source choice, it needs so much re-writting to be next-gen enabled that writing new one should be as effective. From all engines of this generation [even though Source started as past-gen engine], this one is the most open world/big levels unfriendly/mmo and its doesnt even have most next-gen features by default.
Really weird choice, the only logical explanation for it would be, if they aimed for Xbox 360 and 60fps from the beginning, but finally decided against, but it was too late to change engine.