Kickstarter backers had the option of digital or physical tiers, and the lowest tier with the physical collector's edition was US$95 (plus shipping, for those not in the US or much of Europe), so more than the Steam price.

Steam Early Access buyers knew when they bought the game it was digital, and knew or could have easily found out that there would be a retail release. If they wanted a retail release they could simply have waited.

There will be an upgrade option for the digital version of the Collector's Edition. Which AFAIK is what a large majority of Early Access buyers would want.