It is not surprising to me that it is the 3-man group that is mounting the tour. They were together as a unit longer than the previous incarnations and it makes sense that they could have the strongest sense of connection with each other. A 3-man configuration is also the most straightforward to put on tour.
If they were to do a tour as the 5-man or 4-man line-up, there are a number of complexities to consider. Would they focus only on the earlier repertoire, which some fans would love? I'm not clear that Tony, Mike, and Phil would want to ignore their later repertoire. They have all talked about how much they like their later repertoire, so I can't imagine them agreeing to not play any of it. Also, if Peter were to somehow agree to a 5-man tour focusing on earlier repertoire, what would Phil be doing, other than back-up vocals? It would be at best a 4.5-man tour
Alternatively, they could cover a broader range of repertoire, but then how would Steve and Peter be involved? Would Steve be OK covering Mike's guitar parts on the later songs with Mike playing bass? Would Mike be OK with this? What would Peter do? They could do something that The Eagles did on their History tour, splitting the show into two parts - first part 5-man or 4-man, second part 3-man. But would Peter and Steve want to be part of a tour in which they only played on half of the setlist?