Doflamingo wasn't even aiming at being a Yonko Making this thread pointless
It's like asking why Crocodile didn't try to become a Yonko he was aiming at capturing Pluton
Nobody becomes a Yonko because they want to, they simply acquire that status once they check off all the requirements and I think Doffy checks them all off. Sure, his individual strength may be lacking compared to others but I dont think that's what makes an Emperor.
Kaido said it best: "You were just too fucking weak, Joker."
Half the SHs wiped out Doflamingo and his entire crew without even going into tryhard mode. Luffy's basic G4 alone was enough to completely outclass Doflamingo.
Now let's remember how Luffy fared in his first encounters against actual Yonkous (heck, even Katakuri, a mere commander) ... you should be easily able to answer your question yourself then.
Perhaps but if Buggy ends up becoming a Yonko(totally can see it coming), individual strength isnt really a reason.
Not a single one of the Emperors became one because of individual strength.
It's not half the SHs... You forget how many people participated. 5 Supernovas(Zoro, Luffy, Law, Cavendish, Bartolomeo), Chief of staff of the Revolutionary army with couple of his men, fighting King Elizabello II with his famed punch, other captains of the SH Grand Fleet like Don Sai who overcame the legend Chinjao(who also helped), then Leo, Ideo, Hajrudin the giant, commodore Orlumbus. two Scabbards...
Sure, some of them werent going all out and were tasked with pushing a plot elements but still, Doflamingo was facing 2 Supernovas.
Sure, Trebol was on his side but aside from occasionally holding them down, he didnt do anything. Facing two of the Supernovas that are facing the Emperors now, pretty much close to the same odds. With his internal organs wrecked by Gamma Knife...
Would one of the Emperors do considerably better against Luffy+Law if they had their organs shredded? Perhaps, perhaps not...
Surely it looked like G4 was outclassing Doffy in every way but that was after Gamma Knife which noticeably lowered his performance and yet, he endured through it and outlasted it. Let's double Doffy's strength, do you think that would have given him an Emperor status?