Paris is ancient and built on a bunch of hills, there are quite a few places where retrofitting the city to make it more accessible would be easier by rebuilding the whole thing than by making changes and that's not going to happen. Amsterdam has an easier time of it, not quite as old, though old enough that that isn't a big factor in the difference, the good bit is that it is mostly flat (other than the canal bridges, some of which can be pretty steep). Even so there are areas of the inner city that would be hard to navigate in a wheelchair.