I think because B5 had already a story to tell from the beginning while DS9 was a setting at first.
I doubt that the changelings and the dominion where planned from the beginning.
There's an interview somewhere indicating they didn't come up with the Dominion until the second season, explicitly saying they put the first reference to it in a Ferengi episode to mislead and surprise viewers.
There's an interview somewhere indicating they didn't come up with the Dominion until the second season, explicitly saying they put the first reference to it in a Ferengi episode to mislead and surprise viewers.