Why are Christians so conservative when Jesus was like the most radically progressive guy ever?

It's a coincidence.
As you point out, Christianity is not a predominantly conservative ideology. It's not radically progressive either. Or maybe it's both at once. The apostle James sums up Christianity as: "Care for the dispossessed out-groups" (very progressive), and "be morally pure" (very conservative) (James 1:27). I don't think this is an inconsistency, as much as the internal stress that keeps true Christianity strong and meaningful.

Now all that goes out the window when Christianity becomes an easy means to gain social and political influence, which is where we stand in the US in 2020.

Political influence:
Groups who enjoy power/influence have a predisposition toward conservatism. If you're in power, it's in your best interest for things not to change. White evangelicalism is a powerful institution in the US, so it's practitioners are in no mood for change. If you want to find progressive christians, look in the places where being a Christian gets you no political capital. (so, probably not the US).

Social influence:
One of the hallmarks of conservative ideology is that moral purity is important, and can be attained through rigorous control of the body (whether your own, or everyone in society). Lazy, legalistic Christianity maps perfectly onto this part of conservatism which has led to 2 major problems:

  1. Power-hungry conservatives (whether christian or not) co-opt Christianity to claim the moral high ground in every debate.
  2. Christians get convinced by their pastors and the GOP that if they care about morality at all, they must be and vote conservative.

The ages will not look back on the unholy union of Christianity and conservatism with grace. It's a real shame and does damage to people inside and outside of Christianity. Jesus is not proud of this.

Further resources:

source: am a white evangelical.

/r/TooAfraidToAsk Thread