I don’t have a lot of experience with this, but I’ll give your question my best shot!
The key point is hexagons are strong. They tessellate (fit together perfectly without leaving any gaps) and are much stronger than other shapes under compression. This means that if you try to squash a grid made up of hexagons it’s harder than with other shapes. This helps nanotubes to be really strong despite only being a single atom thick!
They also have a large area inside when they are joined in a grid compared to other shapes and have fewer grid lines.
I hope that answers your question!
Hexagons and hexagonal grids are also common in nature. Bees use hexagonal shapes in their hives because it’s strong and they don’t have to use as much wax to make it. Giant’s Causeway is another great example of a hexagonal grid in nature.