No, it's because of the nature of the
CFA.
You can either read every pixel (3x crop), or skip lines* and read every 3rd line (non-crop mode). You can't skip non-integer number of lines (obviously), and you can't skip every other line because you'd end up with either only red and green pixels (no blue data) or only blue and green pixels (no red data).
I suppose you could skip 1 of every 3 lines (read 2 lines, skip 1, read 2, skip 1 etc.). You'd have pixel data for every color, but you'd end up with a really strange bayer pattern, that you'd probably have to write your own custom demosaicing algorithm. This would also require extensive reverse engineering (we don't know how to implement custom readout schemes like this on CMOS or if it's even possible => my bet would be it's not even possible).
You could also theoretically skip differently horizontally vs. vertically (you could read every column, but only every third row). This might be useful for anamorphic lenses. But again, this would require custom readout mode which we don't know how to do, or if it's even possible.
*There is evidence that the 5D3 bins pixels instead of skipping them, but the limitation remains.
