The problem with a sandbox for a declarative platform.
An in-browser trial environment is easy to do badly: you prepare a fake domain, show three screens, and whoever looks learns little — because the interesting part of Pwas isn't the interface, it's what happens when you change the domain description and everything else adapts.
A useful sandbox therefore has to let you genuinely edit the declaration and see the effect: add a field and find it on the screen, change a permission and watch a request be refused, ask the agent for a disallowed action and watch the plan fail to compile. That requires real, isolated execution per session, not a simulation.
It's feasible and it's in the works. But we'd rather publish it when it works that way than ship a version that shows screens and teaches nothing.