Vertical has been THE only way to store LPs since the stone age. I do not know why, but it may be due to 78' era discs. 78s were extremely brittle, and could NEVER be stacked because the weight of them on top of each other would always break the bottom ones, always.
So, as a historical need, it became a 'this is the way it's done' and passed into history as the only way to store LPs.
Aside from ease of removal, no reason for NOT using vertical storage I know of.