Because there was a debate about whether an income tax was “uniform” (see the second part of the Constitutional text quoted above).
Exactly, so it doesn’t really grant the power to tax anything. Adding a new tax can even require amending the Constitution. I would argue that instituting a Federal carbon tax should require a Constitutional amendment. How is it apportioned? Is it based on your body size? The length of your commute? The size of your house? The type of your car? Where you live? The 16th amendment allows unapportioned taxes on income, but CO2 output isn’t income, it’s exhaust. Of course there will never be an amendment to make this legal since it would never pass. But that’s no matter, soon there will be a little CRBN field on your paycheck stub next to your MDCR and SSDI. Why bother amending the Constitution when you can just ignore it? That’s what they’ve been doing with wars for more than 50 years.