The main thing is that their frontend was built for whatever APIs their own platform supported. You can theoretically adjust the code to use the Mastodon API instead, but it can actually be kind of a tedious process to change everything over.

It's probably easier to mod an existing frontend to look and act the way they want, or write a new one from scratch.

The other thing is, it's still a really new instance, and kind of started as a community experiment. While it looks like it could be a big next step for Pebble, they're probably more interested in testing the waters before doing any serious committed development on it.