I think it may have been a political choice. Fireworks.ai is apparently providing compute for Stability for SD3, and Fireworks has had https://fireworks.ai/models/stability/sd3 up for a while which appears to be basically the same thing.

It's an odd thing going to Fireworks and getting a page where you have to provide a Stability API key.

I get a feeling there may have been some agreement for Stability to not provide a UI to drive traffic to Fireworks, and it went south for both of them because no one could find the Fireworks page to send traffic to and adoption of SD3 was lower than expected because of the lack of a UI.

That being said, it's very easy to wrap in a CLI. I'm using a ~300 LoC Go CLI application I wrote to call the SD3 API, download the image, embed the prompt in the EXIF data, save it to disk and then open it in Firefox so I can see it.