It may not be a model quality issue. It may be that GitHub wants to sell a lot more of Copilot, including to companies who refuse to use anything from OpenAI. Now GitHub can say "Oh that's fine, we have these two other lovely providers to choose from."

Also, after Anthropic and Google sold massive amounts of pre-paid usage credits to companies, those companies want to draw down that usage and get their money's worth. GitHub might allow them to do that through Copilot, and therefore get their business.