Only one reason really, PoW mining expense
This. If mining costs more than bitcoins, miners actually take bitcoins out of the system to pay for their electric bills.
So if bitcoin has a market cap of $1 billion and electricity costs are $500 million, miners will actually pull $500 million worth of money out of bitcoin.
Amazing huh?
And if the electricity costs are over $1 billion...the cost of bitcoins will go negative and anyone using bitcoin will actually be paying to use it.
Or, more realistically, if miners are selling all of their coins there is a built in maximum inflation built into bitcoin which can easily be eaten up by the influx of new users.
Mining cost is not static though, from what I have observed over the last 4 years, mining cost roughly always trying to approach 10% of the Bitcoin marketcap, that seems to be the equilibrium point. So no, mining cost will not be at 50% of marketcap and it's impossible to be at 100% or more.