Um, not sure. But I think I may have an idea why. Some tractors do not have an actual roof (not sure if I'm clear enough), so there is no closed spac for you to be in. Also, just to be sure, since I didn't find anything in the dictionary about it, I searched it up and on a tractor gets way more hits than in the tractor. In fact, the results for the second one are really few :/
That's just my opinion though. I'm a native Greek, not English. If anyone could explain it better than I do, that would be nice, just to be sure^.^
That's what I would have said. Yes, I also used "Google BooksNgram Viewer" which showed a huge difference in favor of on.
most tractors in England now have a roof, so we would say sitting in. I think both in and on should be accepted