The upward tilt of combs is likely a reflection of the fact that honey bees
build their nests within enclosures, and not out in the elements.
Moreover, honey bees do store excessive quantities of nutrients, and
increased carrying capacity would favor survival.  Hornets and wasps tend
to abandon the nest, without any storage.

All the wasp/hornet nests that I've observed have a vertical orientation
(that is, nest is built vertically, with stacks of comb on top of one
another), with combs facing downward.   Wasps build nests naturally under
large limbs of trees, under rock outcrops - not totally protected from the
elements.  The requirement seems to be more that the nest has a good strong
surface to be attached to, not necessarily being out of the elements.

A downward facing comb will prevent moisture from entering the brood comb
and suffocating the larva.  With a downward facing comb, a nurse wasp would
just need to break the surface tension of the drop of water to allow the
water to fall to the ground, rather than being forced to somehow "bail out"
the water in an upward tilting comb.

Seems obvious to me that honey bees evolved from ancestral wasps, and
selection pressure slowly began to favor the storage of nutrients, good
nest site selection; selection pressure favored traits that would increase
the amount of storage capacity/efficiency.

  The only consistently downward facing comb I've observed from honey bees
are queen cells. It's interesting that even if ample room is available for
horizontal queen cells, the bees will generally still build queen cells
facing directly downward, just like wasp/hornet larva.   Perhaps there's a
fitness advantage for larva facing downward that was abandoned in favor of
increased storage capacity.  This would be an "evolutionary trade off".

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