Pretty simple really: Blackbeard is just more interesting.
Imu is just a bigger / badder version of the same type of villain we've got for the entire series.
We find out there's someone hidden in the shadows, controlling everything, they see themselves as a god and are evil as shit. Where have we seen that before?
Crocodile, Enel, Doflamingo, Orochi, Kaido and so on...
Imu is the same thing again, but just in a world scale. He is powerful and has a lot of control, of course, but it's just the same again. It's boring.
Blackbeard is unique, he is an antagonist who has been doing shit for a long time and has been fucking up Luffy's life indirectly. He caused Ace to be captured, he caused the war which lead to Ace's and Whitebeard's death, dude attacked Hancock and captured Koby, now he is fucking with Garp, Luffy's grandad.
Blackbeard is also a mystery, dude can eat multiple devil fruits, we don't really know his motivations or his past.
Blackbeard is also a weird fighter, he gets hit all the time and cries in pain, he isn't a badass stoic monster that will fight Luffy like most, he would jump Luffy and the Straw Hats with numbers and scummy strategies, that would be different and cool to see.
That's pretty much it, Blackbeard is just a lot more interesting, he is a very unique antagonist in the series, while Imu and Gorosei just feel like more of the same, just bigger and badder.