It's because of a lot of similarities. Similar hairstyle, hair length, same hair color, similar bunny-eared hair ribbons, both wear thighhighs, and their normal/most common outfit is a serafuku or serafuku-like outfit.
*Kasasasa*
(bug noises)Oh?!*running*
*running*Escort missionCommander's RoomDon't let a single one escape!!You run fast, don't you?A budding friendship!*Kasasasa*
(bug noises)Tch, what a lucky bugger, nano desu.If it's footrace then I won't lose!