Bernie Sanders and Ted Cruz have become the most visible exponents of their respective ideologies, socialism for Sanders and free enterprise conservatism for Cruz. The CNN debates are a circus. But they're still far better than the miserable spectacle of the actual presidential debates in which the GOP candidates were encouraged to launch petty personal attacks at each other and the Dems were encouraged not to get in Hillary's way. And an actual debate about issues is now so exotic that it actually qualifies as entertainment.
But the trouble is that Bernie mostly has no idea what he's talking about. There are lefties who could give Cruz a good debate. Bernie is just going to rely on his "Why do you love capitalism so much that you want kids to die" and "We have so much money, we could buy everyone a pony" shtick. He's also poor with numbers and is better at quips than policy. And that's what you see here.
After claiming to be a Democrat during the election and a Socialist much of the rest of the time, he can't explain the difference between Socialists and Democrats on taxes. (Or chooses not to, such an explanation would inevitably be unflattering to one side or the other.) But it's not as if there is a difference anymore. And that's Cruz's point.