Two structural reasons.
1. Obsidian is a single-user editor. It does not have the concept of "agent A drafted this, agent B promoted it, the team approved it." The promotion flow needs a state machine that lives outside the editor. a plugin can simulate it but the source-of-truth has to be a process the agents talk to instead of a vault file.
2. Agents need an MCP surface. An Obsidian plugin API won't do. /lookup, entity_fact_record, notebook_write, and team_wiki_promote are MCP tools the agent runtimes call directly. Obsidian's plugin API targets human users and the Electron app. You would be reimplementing the MCP layer to bridge.
Practical compatibility: you can absolutely point Obsidian at ~/.wuphf/wiki/ and use it as a vault (we got someone from our Reddit post do this). Obsidian can be reader while WUPHF stays the writer.