Not an absolute answer but maybe because Luffy's going at them with brute force and it's harder to break something than it is to cut it. Also Luffy broke a biscuit so just because it's the strongest he's seen(which that translation is in debate) doesn't mean it couldn't be broken by someone with comparable or greater strength.
Cracker easily cut (half into) Luffy's arm, while Doffy's string attacks (like 'athlete' to the side) that also cut were bounced right back without effort
You also said that zoro can cut cracker
Cause Zoro can cut anything (like he said to Pica, he even cut Gold clad in haki, though that doesn't prove that he'd be able to cut someone worth 850m bounty, i think Zoro would be able to, that doesn't automatically mean he'd win
Doffy being able to win of Cracker cause Zoro can cut Cracker isn't realistic, Doffy's attack 'athlete' was effortlessly bounced back by Luffy's form only, while Cracker with the first hit cut (half into) Luffy's arm, Cracker doesn't just have a biscuit he has infinite Knights and his own haki is proven stronger by the statement i just did (also by Luffy's mouth)