I can give you that answer. They started as a European company, and went with the metric system. And 30mm is a nice whole number. Any way you can increase the bearing surface for light to pass thru, the more brighter a scope is. But with todays coatings, you can almost perfectly match the index of refraction with air. Different air densities change with altitude and pressure, and humidity so they are not always perfectly matched. But the internal lenses should be constant.

Solitary is correct. Its a wash on a properly made scope that is adjusted to gather all the light possible from the objective lense. Problem is perfectly adjusted. You can have much more slop with a 30mm.

The following is taken directly from Zeiss's website:

"Center Tube Diameter

The diameter of a scope's center tube (or main tube) impacts the overall strength and durability of the scope. And it obviously determines the size of bases and rings required for mounting. But beyond that, the center tube diameter must be adequate to allow a sufficient range of windage and elevation adjustment. Zeiss riflescopes have either a 30mm or a one-inch center tube, depending on the model."

There's no mention of light transmission because there's virtually no difference. As bc said, Europeans being on the metric system adopted a nice round metric number, 30mm, for their standard main tube size. We adopted a nice round Imperial number, 1 inch, as our standard. It's really no more complicated than that.