A wise person on fedi persuaded me that the Democrats (at least, the currently-ascendant faction within it) would rather lose the election than raise those taxes.
If they raise the taxes, they reduce the power of the hyperwealthy, which to them is bad.
If they let the Republicans win, then the electorate will be punished brutally for daring to not lick boot, and will crawl back to the Democrats at the next election anyway.