Is Bitcoin primarily a medium of exchange or a store of value? No financial instrument can be both things equally well. Despite what some Bitcoin proponents believe, specialization in one direction is inevitable (many people who seem to think it can somehow be both are the same ones advocating 'buy and hold forever'...).
I disagree with your premise. Bitcoin makes a fantastic medium of exchange (best in human history), and due to the predictability and certainty of mining new Bitcoins (at predetermined rates), and the lack of monetary inflation, it will also make a great store of value.
Please explain why one cannot both save and spend in Bitcoin?
Price is too volatile for it to be used as a medium of exchange. In my experience ordinary people only use bitcoin as a medium of exchange when they have no other alternative. For example because other payment systems are not as well developed in that part of the planet.
Bitcoin's deflationary nature discourages spending and encourages saving making it more suitable as a store of value.
So, yes, it can be used as both and some people will use it as both but it will primarily be used as a store of value.