Say Arizona doesn't have universal healthcare, and California does. Someone gets cancer, so they move to California. Under the privileges and immunities clause, California generally cannot say "hey, you just moved here, so here is a bill." Consider also a company that doesn't like the extra taxes used to support universal healthcare. So they hop across the border to Arizona, and continue selling to California customers. California cannot, without violating the Commerce Clause, say "hey, you get the benefit of selling to healthy Californian consumers but you moved just to evade the tax that pays for it, so here is a tariff on your products."

The Constitution imposes an open market on commerce and movement within the United States. The Commerce Clause exists to give the federal government the power to address the economic issues that arise as a consequence of that.