There is a massive difference between stopping all companies trading by enforcing national ownership of a resource and stopping foreigners trading resources that are legally on the market.
The question posed was not:
"Why are we selling something considered a national resource?"
It was:
"Why are we selling something considered a national resource to foreign owners?"
edit - also, I fully support mixed market economies with nationalised infrastructure, I think they seem to get more benevolent outcomes, but not with protectionist markets on the bits that are privately owned.
I agree with that, yeah (especially the last edit). I think the "national capitalism" approach, where things are privately owned but preferential access is given to important domestic corporations, often ends up mixing together the bad parts of public/private ownership rather than each of their good parts.
The question posed was not:
"Why are we selling something considered a national resource?"
It was:
"Why are we selling something considered a national resource to foreign owners?"
edit - also, I fully support mixed market economies with nationalised infrastructure, I think they seem to get more benevolent outcomes, but not with protectionist markets on the bits that are privately owned.