well mostly because of this:
The adoption of Bitcoin is apparently mathematically chaotic in the sense that certain small changes to the present situation lead to large and unexpected consequences. There is simply no precedent for Bitcoin, and therefore we depend upon argued imagination.
Well, if there is a risk of large and unexpected consequences, wouldn't it be better to make a more simple change rather than a more complex one? It seems to me that a complete shift to PoS would be simpler than merging PoW and PoS, though that is just my intuition, I have no technical knowledge of cryptocurrency.