An answer to your question might be a matter of perspective. You say conditions are getting worse (note: for really worse, see Britain). To Leftists, conditions are getting better. The Revolution began circa 1964. After six decades of breakthroughs - wins each unimaginable to mainstream Americans a decade previous - the Left is accelerating the pace. Ahead, perhaps when they next win the Presidency and Congress (a timely recession would do it), they’ll begin the Pursuit Phase of Battle. Running down and destroying a broken fleeing foe.

Mao gave a briefer answer to your question: the Left wins because they know “A Revolution Is Not A Dinner Party. …It is …an act of violence.”

The Left has unifying doctrine, organization (including funding), a Broad Front of Alliances (everybody likes winners), strategy & tactics, and a corps of activists (including street thugs). The right has none of this, in any significant form. Do a Correlation of Forces chart. No need to consult Nostradamus on how to bet.

I recommend fear for the right. It’s an effective motivator, and warranted.