DRG is generally seen as the weakest melee. No CC and its DPS phase *increases* damage you take - and good players aren't all going to stack up so that you can debuff all of them with your roar. Not to mention, the LB has a lengthy charge and is generally quite easy to counter by guarding or just... moving away from it. It *can* work, but against high skill players the deck is stacked against you. You would basically need to build a team around the DRG, building to enable their LB to land like it needs to, to protect the DRG when they're in Life, etc. They have great DPS and their LB is incredibly strong if you can guarantee the dunk, but they're fragile and offer no CC.
SAM is similarly not a popular pick. High skill players tend to not attack into Chiten if you have or are about to have LB, and while they don't have any major things they lack, they also don't really excel at anything. They're basically Mario. Not bad, not good, basically just the baseline. But you don't see too many people playing Mario over more specialized characters, do you? Great class for stomping newbies on your way to into Crystal, though.
At high skill, MNK, NIN, and RPR are the most common picks. MNK is tanky and has the highest sustained DPS of any melee and the ability to 100-0 casters and healers is incredibly valuable. NIN is... well, NIN. Top tier poke, great at harassing people trying to potion, and the best sweeper in the game with a toolkit that can do just about anything. RPR is tankier than most actual tanks, decent sustained DPS, and one of the best LBs in the game on the shortest charge timer. But RPR does require you personally understand how to play the class, and it does tend to perform better with teammates that understand how to play the game properly. Tenebrae Lemurum is probably the best setup ability in the game, and with such a short charge timer, it's also really good at peeling in a pinch. And because of the forced (slow) movement and guard break, it has obvious synergy with environmental effects as well.