So Why Is Christianity Not Leftwing?

The simplest answer of all is that nowhere does Jesus ever advocate any form of socialism.
Try as much as you like - you will not find it!
He does warn against the dangers of trusting in wealth and selfishness. Quite rightly! (Hardly a socialist clarion call!)
He does not demand that the state should take over your life and your finances.
He does not undermine God.
He encourages people to build a relationship with God.
He does not undermine the social structure.
He does not advocate uncontrolled behaviour and passions.
He does not attack the social status quo.
He does not undermine marriage.
He does good without ever being a do-gooder.
He never advocates spending money that you do not have.
He cut through the false ideologies of His day and showed people 'The Way'.
He advocated 'trust in God rather than trust in systems'.
Never once does he suggest that tax burdens should be excessive.
He would clearly never have advocated 'the printing of money' to solve problems.
He even supports the idea of investment - He even tells a parable about it. (And yes, of course I am aware of the underlying spiritual meaning!)
He demands compassion for others whereas socialism only ever pretends to care.
I also recall that The Apostle Paul stated that if a man would not work - he should NOT eat!
This runs contrary to all modern socialism - Woy Jenkins even said that drones should not have to work if they didn't want to!
Socialism in the Scriptures? - No.
Some will point out that the apostles 'held all things in  common'. YES, but vitally - they were VOLUNTEERS!