From the Steam Frame purchase page FAQ:

How do I buy the Steam Frame accessories?

First-party accessories (the PSU, Accessories Replacement Kit, and Ergonomic Accessories Kit) will only be available to customers purchasing their Steam Frame as part of the add-to-cart process.

For the PSU, that policy is fine. I wish it didn't exist, but it's... alright. A user might change their mind after they use the device for a while and wish they had purchased a PSU, but many users probably know at the time of purchase if they'll want one. Furthermore, anyone buying a PSU may as well buy one of the many third-party options available (whether purchasing it along with the Frame or purchasing well after the fact).

The ergo kit is a little harder to swallow. Many users may be able to predict that they'll need it, but many users will only determine that it is necessary after using the device for a while and realizing that the head strap is not that comfortable, the hand straps would be useful, etc.

But for the accessory replacement kit, no one knows they need it until months after using the device and their face cushion starts to wear away or something. Being that customers are only able to purchase that item by adding it to their cart along with their Frame purchase, that means all customers will have to make a prediction as to whether or not they will use the device enough that these parts will eventually need replacing. They also have to predict how much wear the stock face pad will take. They also have to estimate how long the replacement pad will last and consider actually purchasing multiple accessory replacement kits along with their Frame?