Labor in the US is pretty expensive in the grand scheme of things. $15 is peanuts to make, but a lot for parents to cover.
Take this scenario. You have an infant room with 8 kids and have 2 teachers in a LCOL area. Tuition is 325/week. Total incoming weekly tuition is 2600. If you find 2 people who will work for $15/hr thats 1200/week in labor costs. Plus FICA, insurance, and anything else required to pay per employee. With no redundancy (so no sick/vacation days). Then you have to pay rent, utilities, maintenance, front office staff, and have enough left over for someone find it all worth it.
There is really no way around it - if you want people at the bottom to make a decent wage then customers will have to pay it. And $15 isn’t exactly decent even in LCOL so daycare costs would probably need to rise at least another 33% in this scenario which moves the low end price of daycare from 1300/month to 1729/month.