Another factor in context window is effective recall. If the model can't actually use a fact 1m tokens earlier, accurately and precisely, then there's no benefit and it's harmful to the user experience to allow the use of a poorly functioning feature. Part of what Google have done with Gemini's 1-2m token context window is demonstrate that the model will actually recall and use that data. Disclosure, I do work at Google but not on this, I don't have any inside info on the model.