They were allied with beast pirates so they dropped their guard, they didn't expect any enemies, and Marco's attack was a surprise attack, he kicked the keel of their ship where they can't reach him with their abilities
Because P1 is still strong for other members of the alliance, even if Sanji is stronger than him. It's a win win for Sanji and Oda. Sanji gets to easily defeat a strong enemy, Oda gets to show off the modified RS and then Sanji can fight someone stronger later.
If Pageone was to be finished by Sanji, Oda would have done that in their last clash. He was casually making Pageone cry his heart out with base kicks. There's so many characters, i am sure he might have him reserved for someone else.
And if you meant unfinished clash mean they will fight, then Sanji vs Blueno in EL, perfect example to give to you. And unlike Pageone, Sanji didn't even overpower Blueno.