I would guess because 入れます is if something is entering in something smaller like going into a cup or box or bag while おきます is to put down something. so 部屋にテーブルをおきます is saying I put the table DOWN in the room while this sentence is saying I put the cup IN the box.
That is a good way to remember it. Just note that size and direction do not really matter actually. e.g. Add salt to the soup would be スープにしおを入れます/put salt into the soup. Leave something outside would be 外に置きます.