One reason that comes to mind is that it would just allow the rich to become richer by providing a cheap workforce subsidised by the tax payer. Imagine if a company wanted to pay less than minimum wage, hire a bunch more people and have their new workers' pay topped up by a negative income tax. It'd just be getting people to work for the sake of working while tax payers subsidise profits for business owners.

Because people who work have money already, people who can't work don't.