They aren't.
Oda screwed it up with Big Mom and Kaido in terms of execution, but the emperors are conceptually stronger. First, because of their variety: a greedy witch capable of giving life, an ogre able tu transform into a dragon, the only man known so far to have eaten two devil fruits... On top if this, they aren't individuals but organizations, each of them thematically different; admirals lack this since they are tied to the Government both in terms of "crews" (Marine soldier with scarf and cap #356) and scenography (boring white bricks). Compare this to Big Mom, for example, who gave us a whole nation made of food and very distinctive subordinates in a fairy tale context.
Yonkos have this feeling of being bigger than admirals, too. We are in a piracy story, first and foremost, and one way or another the emperors have been more relevant than the admirals throughout the story; the latters, at the end of the day, are bound to be defeated as secondary villains (bar Akainu, hopefully). In fact, the moment the admiral breaks from his title or shows some kind of resistance to it (Kuzan and Fujitora), the moment he gains interest as a character because, otherwise, they're just very powerful soldiers.
Admirals lack the mightiness of emperor's too. Whitebeard was the strongest man in the world who died standing, Blackbeard is a D. and will likely inherit Xebec's will, Shanks is the main character's idol... The whole idea of being "great pirates" ruling empires is, in terms of romance, way more powerful than anything the admirals can achieve.
Oda stated his original idea was for One Piece to be about defeating the emperors, and you can tell. Maybe he didn't execute as good as he could (this applies beyond emperors and we will see how he treats admirals too), but it's obvious that the amount of work and creativity behind the Yonko completely surpasses the admirals; which is, in my opinion, what makes them more interesting: emperors feel like differentiated concepts and important actors in the world, while the admirals are an elevated CP9.