Because polarized lenses cost more to make. Which means Zinni has to pay more. Also, depending on the Rx and frame combination, they can be a little more complicated to manufacture as well.
If you have a reasonable Rx, Zinni are likely using finished lenses. Polarized have to been surfaced in the lab.
Also, in my experience Zenni doesn't necessarily make the polarized lenses to adequate tolerances. I ordered a pair of aviators with polarized lenses and there were such severe pressure points around the rim that it distorted the polarization across almost the entire surface of the lenses. It made the polarization coating basically worthless, and I had to return them to order a non-polarized pair for half the price.
Yes. If the lenses are too large (circumference) too much pressure will be placed on the lenses and they’ll do just as you describe. Certain materials (polycarbonate) will actually crack in the same situation.
If you have a reasonable Rx, Zinni are likely using finished lenses. Polarized have to been surfaced in the lab.