There is at least some overlap in the form of those who care about both canon and ships (me). I would suspect, though, that at least some part of it is how canon is read. Naturally, when reading with an eye for creative fanworks, one will look for possibilities of what could be. Strong possibilities in the form of established relationships that would be compelling or seem particularly close to changing in form naturally provide an attractive jumping off point. From there, the most popular ones easily become accepted fact though hopefully with an understanding towards those who don′t ship it. Meanwhile in the synthesitive side of the fandom (which is no less creative, just in a different way), all that matters is what is in the text. Rather than building out, ideas are built inwards, exploring every detail of what is already there. Since they are built from the same foundation but the former is much more loose with it, it′s natural that disagreements about that foundatin would form. And then, since it is a foundation, such disagreements can be seen as a threat to the whole of one′s work built off of it.
I could point to all of the scenes that in my eyes speak of romantic possibility and then I′m sure someone here would be happy to play the role of pointing to scenes that in their eyes speak of platonic fact. We would then after a bavk and forth leave, frustrated with eachother.
I′m sure I saw a video essay about this phenominon some time ago, but I am too lazy to dig it back up.