They're being told to by management. That says approximately nothing about the relative merits of the two products, but it certainly says something. What, exactly? Who knows...
Other companies have a similar top-down "use Claude" mandate as well.
Is it nothing though? How many Apple employees do you think use Windows? And how many Anthropic employees do you think use GitHub Copilot? I would assume the answer to both is approximately 0.
Citation needed.