If you just tax carbon emissions at say 1$ per ton it's easy to track. But with cap and trade people who say plant 1 million trees in the desert can get credits for making things cleaner and sell them. Now the plants all die, but that's not important as long as you can convince the regulators that some of them might make it your in the clear.

With a simple tax everyone pays, when it get's complex those who understand the system pay far less.