Because the more income you earn, the higher your marginal tax rate and that means you'll pay more capital gains tax in those years. (moneysense.ca)
I make a lot more capital gain from my gallery practice than I do the public sculpture commissions I have won. (fadmagazine.com)
And most mutual funds, except index mutual funds, are actively managed, potentially generating more capital gains and thus more taxes on them. (freefrombroke.com)