Because Roger's final enemy were Rocks, Kaido and BM.
Roger, Rayleigh and Garp were strong enough to handle them. Plus WB joined him.
But Luffy has to fight BB and his crew. Every one from BB's crew is far more superior (If not 100% in brute strength then in cruelty and cunningness) than Luffy and his crew.
How is ussop supposed to fight that sniper from BB's crew if all he can do is run with his penis tucked between ass cheeks?