Cost and availability. Cylinder filters tend to be way more expensive ($30-50 per, I've seen true HEPA cylinder ones "for allergy sufferers" aka medical space,

Cost and availability. Cylinder filters tend to be way more expensive ($30-50 per, I've seen true HEPA cylinder ones "for allergy sufferers" aka medical space, >$130) than rectangular (a few bucks). Rectangular also comes in a wider variety of form factors than cylinders, with more different filter styles (eg different thickness, with carbon, high flow, true HEPA/MERV >13).