Because he's exciting to watch, simple as. Not many wrestlers bring so much energy to the ring like he does, he could pull off exciting moves, like his diving crossbody for example, it's a really beautiful looking move especially when it's replayed in slow motion. I don't get all this "he botches a lot" argument because he rarely botches now, and also his style might be different from others but i actually like it, i don't see why i should complain about his wrestling style as long as he and his opponent in the ring can produce an enjoyable match, isn't that the point after all? To be entertained by a wrestling match? He could consistently produce enjoyable match on weekly basis, he's actually one of few bright spots in the overall dull year for Smackdown last year.
A lot of people say that "he's generic as fuck!" and "he has no gimmick!", well, blame the creatives, most babyfaces in WWE are like that, but at least he does seem like a genuinely nice guy on the screen and not faking it, he's also a very charismatic person and connects to the crowds like no others, so it's no wonder why a lot of people in the crowds cheer for him, people just can't help but chant for his name and encourage him to make a come back everytime the heel is dominating him in the match, he's just a very likable guy to general audience and i feel the same about him. Lucky Cannon, who also portrays the "babyfaciest of babyface babyfaces" character, is also "generic as fuck" like you might say, but the difference is, everytime Lucky talks i just want to punch him in the face, i still lol at his sappy life story promo at that one episode on NXT, he was supposed to gain sympathy from the crowds but he just got booed instead, that's what happens when you can't work the crowds well and can't pull off a good babyface character.
Oh, and i don't think he's bad nor average on the mic, i know he can deliver an entertaining promo if he's given a better material than pandering to the crowds stuff like he usually does because he's already done it before, take his promo when he destroyed Orton's car for example, it was just plain awesome, he could show that he could be funny yet badass at the same time, it also just shows you that Kofi can do BETTER than this "goody goody" character he portrays, it's just WWE never gives a better promo material for him when he's on the mic and never gives him any opportunity to develop his character, they tend to play it safe because Kofi is already over with the crowds so they keep him stuck in his current babyface persona.
But yeah, all in all, i like him. Looking forward to see how his IC title run will conclude, who knows once he loses the belt to other midcarder someday he'll get a main event push, and i hope he will.