Originally Posted by senatorsca

Main reason is that gravity works against the eyelets vertically, however when mounted on top of the tube, the majority of forces are lateral. Failure of an eyelet when mounted on top would lower the probability of a catastrophic failure that could eject the battery from the bike at high speed. Considering that the battery is the most expensive part of the system, usually, you really want to mitigate any of those possibilities.

Also in case of a crash, it is exposed and is not protected by the frame from impact below.