High-income people are frequently wealthy (outside of incomes derived through lottery winnings), but the two are not equal in their political views - those sitting on their wealth will be in favor of income taxes (since they barely have any), those who work for income will lean in favor of asset and consumption taxation (property, estate and sales taxes).
However, the anecdotal evidence of "stored wealth vs current income" tax is there, notice how "let's-back-tax-everybodys-income" Prop-30-happy California is also home to "dont-you-dare-to-raise-taxes-on-my-mansions" Prop 13. My original remark was to point out that wealthy people and high income people are frequently advocating opposite causes.
Mansions? Hardly. More like "Don't raise taxes on my tiny tract home through the roof based on ridiculously inflated real estate values."
Maybe in a state that didn't have both high sales taxes and high income taxes a cap on property taxes wouldn't make sense. But in California it does.