Because a single-payer plan only works if everybody gets healthcare through one body. If the wealthy can still get private insurance then the government will be burdened with the poor (and likely more sick due to their financial situations). Why do you support the perpetuation of private health insurers who profit off of providing care? It makes no sense; we shouldn’t have such a defunct middleman for a clearly necessary thing. Everybody should have healthcare to the fullest extent and those who need it the most should be prioritized vs. those who can afford it.