A couple of theories: You’re putting capital at risk, so you need incentives to account for the risk. Another is that capital gains aren’t indexed to inflatio

A couple of theories: You’re putting capital at risk, so you need incentives to account for the risk. Another is that capital gains aren’t indexed to inflation (which would be a nightmare to compute), so the long term rates are lower to bake inflation in. The US used to tax capital gains at higher rates, but when they did they also had generous exclusions (IIRC, the first 50% wasn’t taxed)