If I remember correctly this was brought up in a live letter about a year ago. In short, its both reasons. He stated there's a bug that when you interact with an NPC while mounted the NPC starts glitching so they made it so the character auto-dismounts; he also mentioned he's aware that some people may block NPCs with large mounts which can be annoying for many players. At that point in time it didn't seem like something that would be a clear benefit to the game or players so he wasn't inclined to devote resources to fixing the bug.