As mentioned the problem is that the element needs to be covered by liquid while in use. By going in the side as the BK is covered around the 2 gallon mark, you can start heating getting it just below the boiling point. Then once you are up to full volume you only have to get it to rise a couple degrees to boil rather than having to wait to heat the full volume.
That said there have been some people who have installed them through the bottom the benefit being it is easy to get a whirlpool going without the element getting in the way.
