Part of the answer of why we can’t have this here is that Seattle isn’t New York: it simply doesn’t have the density, the transit system, or a population that has largely escaped the mindset that their car is the only way to get around.
But another part of it is institutions. Our local laws simply don’t allow political appointees these kinds of powers. There are lots of rules about process and a tendency to have an election for even minor positions. After all, in many ways, Ms. Sadik-Khan is a 21st century version of Robert Moses, if on a much smaller scale, and was spawned by a similar set of political structures.