Because they are the ones actually paying the taxes.
I am 100% for tax cuts for anyone, anywhere, at any time, for any reason. The only way to fix this mess is to reduce taxation and reduce government spending.
But this plan will raise taxes on the middle class. Why should I support additional tax credits for the wealthy when my taxes are going up? That's why I wonder if this child tax credit expansion is a poison pill meant to derail the whole thing?