The other orientation is much less stable; it’s a stationary point but if nudged will rotate away from droop-up. Similar reason old pre-float-glass windows are

The other orientation is much less stable; it’s a stationary point but if nudged will rotate away from droop-up. Similar reason old pre-float-glass windows are mostly found thick side down; easier to fit that way because they’re more stable. The myth that they flowed over time is just that. :-)