User blog comment:TheStrawHatFollower/Whole Cake Island Arc!/@comment-5132301-20170306212521/@comment-24850639-20170307194917

For the record, I'm completely open to the idea of him becoming one, I'm just following Oda's pre-existing formula. Our knowledge on the Yonko themselves is limited, even on the people we've seen fairly a lot of, like Shanks & Whitebeard. However, an extremely large crew structure and an established territory are the two things that have remained constant. Establishing territory means that at some point, all the Yonko have settled down. They have a "base", or a place that they consider home of their operations while they try to figure out their next move. In addition to that, their crews are so large that there are multiple sections of the crew up to different tasks at any one time. The Straw Hat Pirates have not followed, nor will follow either of these seemingly cardinal rules:

1.) They are on a one-way trip to Raftel. Yes they took a 2-year break, but its because they got forcefully separated and realized that they needed to get a LOT stronger. Other than that, the Straw Hats simply move from place to place that their search for One Piece takes them. They won't be spending a long enough time at any one place that they'll ever be establishing a base or hideout.

2.) Yes the Grand Fleet are allies, but they are not Straw Hats. They still maintain their own indivual crew structures,  independently report to their own leaders, and have their own motives outside of helping Luffy become the Pirate King. Whitebeard did have a fleet of allied pirate crews, but his own individual crew was still large enough for him to have "divisions". That's just not something I see as ever being likely for the Straw Hats. They will have all kinds of allies in all kinds of places, but I don't see the main crew itself ever getting so large that it becomes difficult to keep track of the character development on the individual Straw Hats.

You're right that at the end of the day its the WG who makes the call on who is or isn't one, and they've recognized Luffy's ability to garner hordes of allies on multiple occassions, but he still doesn't influence the world to the degree of someone like Kaido or Big Mom.