In most serif typefaces, one of the glyphs is significantly thicker than the other, and they don't meet exactly at 50% height. If one rotates the letter X 180 degrees, it would look out of place.

https://upload.wikimedia.org/wikipedia/commons/e/e6/Capitali...

there is no XI in XVIIIII though

ah nvadr spells it out:

VIIIIX

https://news.ycombinator.com/item?id=39722131