Because an LLM does not get smarter and it does not learn. It is fully cooked when it launches and only gets smarter with more training of some sort, a long, expensive, process that doesn't get phrased as "we thought the model would get smarter".
However, if it is a scientific experiment, then changing a primary variable in the middle seems like bad thing.
I don't actually think it's a scientific experiment, of course, I think it's a PR stunt. So, they can change the model all they want. Nobody's learning anything from this that you can't learn without burning $100,000.