I think because it would have been more difficult for them to erase an island if it was affiliated to them had Sabo been there.
For thrm it made it easier to kill two birds with one stone, Sabo for them absolutely should be dead, and in case he leaked something anything ti that kingdom who happens to have revolted too.
not just because they revolted, to move such a weapon they need an order from Imu, if they wanted to destroy the island just because they revolted they could have sent a buster call, & they just knew that sabo was there, sabo just happenes to be there, even the gorosei said it was his bad luck, & the other gorosei said it was fate, they didn't account for that.
it's like "we are going to destroy Lulusia".
marines: "sabo is in Lulusia"
gorosei: "it's his bad luck, we were going to erase it anyway"
gorosei: "nah, it was his fate"