Hexagonal packing is the densest way to pack circles or spheres [1] [2].

That's why it pops up all the time in nature, from honeycombs to crystals, the way bubbles, beads, etc arrange, etc.

Of course circles and spheres are also optimal in term of surface area to volume ratio.

Bees have evolved to build the optimal honeycomb.

[1] https://en.wikipedia.org/wiki/Circle_packing

[2] https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres

Octagons don't fit together on their own; they need squares in between them. Like a soccer ball. Hexagons let you cover area with the same shape over and over again.