Very simple. This is the S&P 500 percentage of stocks above 200 days moving average:
As you can see the line is in downtrend all the way from June. The gold line is S&P 500. As you can see there is a negative divergence from July peak to October peak. While $SPX made new all-time high in October only 63% of stocks were above 200 DMA. The July peak was coincident with 74% of stocks above 200 DMA.
We are in Christmas rally (Merry Christmas!) but the line is at 46%.
The longer chart:
The line was mostly above the magic 50% separator all the last 3-4 years. But now it spends all the time there. And you know that for every stock the 200DMA line is a magic crossing that is hard to penetrate both from above and below. There are no enough leaders to push the index up much more. The October high looks like a mount Everest as early as only two months later