Why did the Chinese government refuse to sign the Simla convention?

The Chinese government refused to sign the Simla Convention because it maintained that Tibet was an integral part of China and therefore lacked the sovereign authority to enter into international treaties independently.1

While delegates from Great Britain, China, and Tibet initially gathered to negotiate, the Chinese republican government subsequently disavowed its representative and declined to ratify the agreement. China later characterized the resulting McMahon Line as a product of British imperialist aggression, arguing that the boundary was illegal because it had never been recognized by any Chinese central government.21