I highly suspect it is plain cheaper (and more convenient) to just subsidize private car rides for disabled people than making entire transit infrastructure in huge city to account for them.
Better yet, take the money that government would otherwise spend on building all the wheelchair accomodations, and just distribute that between wheelchair users. I am sure many of them would prefer that over subway improvements which they might never use.
"Disabled people cannot reach their [work, school, childcare, w/e]", if true, is a valid problem. There are multiple ways to solve that problem. "NYC subway does not have wheelchair accomodations" is a useless outrage over circumstantial symptoms.