While an organ has a similar interface (keyboard), it's such a different instrument really, in terms of the noise it makes, and various other elements like the length of noise from a key press. He would have had to actually want the organ as the instrument in the piece for how it is, not just for offering a greater range than an early pianoforte.
So it could well be that he would have preferred to use the organ's range earlier but just not wanted that instrument in his music. And that's not necessarily an insult to pipe organs - I think they sound lovely, watching somebody play well can be as enjoyable as almost any other instrument, and I'm a big fan of them in their place. But their place is pretty niche really - there's lots of lovely (but religious) choral works that use one, due to churches historically, and there are nice things written for organ. But it's pretty rare for organ to be the keyboard instrument of choice in orchestral music, chamber music, etc