This simple answer is it's easier. Income is mostly cash-denominated, so it's very easy to value. Wealth is often illiquid, so it's hard to know how much to tax

This simple answer is it's easier. Income is mostly cash-denominated, so it's very easy to value. Wealth is often illiquid, so it's hard to know how much to tax. Look at property tax, for example. It's a wealth tax and it requires a very complicated apparatus to generate ok-ish numbers every year. But that mostly works because houses are a moderately liquid market, so appraisers can mark to the existing market. And even then there's a lot of dubious stuff around the margins, like the way Trump was giving different valuation numbers depending on whether he wanted a loan or to brag (high valuation number) versus paying taxes (low valuation number).