Because ACA did NOT raise costs on the middle class, or even on the upper class. 
What it did was actually LOWER costs by not having to deal with so many not paying, and then shifted costs increases on over to small amounts of taxes. 

IOW, it is trump that just caused MAJOR increases in our health care costs, not ACA. 

Im not convinced that ACA is the best approach, but it was a LOT cheaper than what we had and what we now have.