Probably the biggest reason is timing. The Aeron came out in 1994 and became the chair to have for dot-com startups. It was a huge perk to have a chair like that (there wasn't really a comparable chair at that time) and that led to enormous brand awareness.

I bought an Aeron a few years ago, but also considered the Sayl. I went to a Herman Miller dealer and tried all the chairs. A salesperson talked me through the different adjustments for each chair and explained why people preferred each of the chairs. When someone can sit in them side-by-side, they will pick the one that they like the most, which might be the Sayl. But many people only know the name Aeron.