Marco was considered a candidate for the Yonkou by the Gorosei. You must have skipped a lot of chapters to come to such conclusion.
Yeah "Considered"
And then we saw the result "Utter Defeat & got Overwhelmed against BB who many consider to be weakest Yonko at the time"
And go check, they didn't say Marco was Yonko Candidate, they said Marco alongside rest of crew could possibly stop Blackbeard.
Sorry to tell you, they failed miserably
Ryokugyu says: "An Admiral can't lose against some Commanders"
Marco: "I can't hold against two Commanders for long"
He doesn't even consider himself close to an Admiral, so no need to overhype him