The differential gain/loss is heavily traded. Since the CME gives a 70% margin discount on the correlation, this trade is highly leveraged.
Remember, if there is a strong correlation among leverageable assets, in this case NDX and SPX, then you can either trade the outperformance of the tech index with a long/short position in NQ vs ES, or you can overweight the SPX and hedge out some vol by shorting the correlated NDX futures.
A high degree of correlation can be used to trade an outperforming asset, and if inversely weighted, the more volatile asset can be used to hedge exposure to the underperformer.