I’m of the opposite opinion. I think that the re-emergence of the OF has contributed to a greater appreciation of the continuity of the Church; debunking the untruth that we have 2 Catholic Churches, pre- and post- Council.
I don’t deny that there have been some problems, but I think overall Summorum Pontificum has been of great benefit to the Church.
I also think that it should be (but that’s not my decision to make, surely).
The Pontifical Commission Ecclesia Dei (who has jurisdiction over this sort of thing) is working on that issue. They did say that new saints (canonized after 1962) can be inserted into the EF Missal, and they are working on inserting the new Prefaces. So, at least in some sense, the calendar is something on which they’re working.
On a local level, the possibility of Votive Masses provides some limited latitude for more compatibility.
Unfortunately, it’s not a simple matter of the Church saying “pray the EF, but use the OF calendar.” Sometimes that works, sometimes it doesn’t. The OF has no Epiphany Season, for example.
So, to answer your question: yes, there is a reason why the two are incompatible. But that doesn’t mean that they are irreconcilable. Hopefully, the Church might someday find a way to make it possible.