Also avoids confusion with lowercase "l" and the digit "1", I suppose. I've seen "I" skipped in various contexts (not just Go notation) for this readability reason. (E.g. some random password or other random text generators have options to not produce 0 & O, I & l, etc...)
What's annoying is that it's not consistently skipped. I can't remember exactly where, but there is an online site I've played at and a .sgf editor I've used that were at odds on this. It wasn't an issue until I tried to manually edit the .sgf file, at which point my brain exploded.