BTS started out as just another BigBang clone but then when they started to gain some momentum they went from a hip hop group to a more tradional style boy group like TVXQ and that's when they took off.
BigBang on the other hand was the biggest boy group in the world in 2015 and went as far as a kpop group could go at the time. But unlike BTS they stayed true to who they were as a group and as artists. Their image and style had a ceiling and it reached it.
BTS decided to change because they knew they could reach a larger audience with their current persona. You can appeal to a much larger audience with Boy With Love compared to Bang Bang Bang. So it makes me take away points from BTS success cause they just morphed into whatever would work better globablly while BigBang stayed BigBang for their entire career.
You can't convince me if BTS stayed hip hop they would be where they are today. I swear everytime I compare these groups I'm blown away at how much better BigBang and their legacy (as artists) really is.