Because a well diversified portfolio is a good comparison/reference for individual investments. A better choice than S&P 500 would probably be the FTSE Global All Cap Index.
Anything after Jan 1 2020 is probably not a good comparison due to the Covid-related market volatility. Anything before could also make sense (personally I have a long term target so I care more about longer time ranges), but would be even more in favor of Bitcoin.
