I really doubt that he has been into this tournament because he is stronger. We have clearly seen how strong goten and trunks are. They can transform into SSJ 3 as well. And also they gave a tough time to buu, who transformed everyone into chocolate. And clearly it was seen that master roshi didn't stand a chance against them. Its probably because of that void thing. Goku might not want to take risk with his little kid. And same goes for Trunks. Also their might be a possibility that goten and trunks stayed back to protect planet earth in absence of allmighty warriors. 

**Edit 1** Master Roshi, may not be strong. Maybe he has been taken into the tournament of power because of his strategic planning and  other skills.