Because Seattle’s min-wage isn’t even up to $15 yet. It’s being stair-stepped up to $15 over time. In April 2015, it was raised to $10 or $11, depending, and this research proving that the min-wage increase hasn’t negatively affected employment rates was done based on that particular wage floor, even though it saw another stair-step increase on January 1. It won’t be for another four and a half years until it reaches its final destination of $15.
The reason it hasn’t been deleterious is that Seattle is a fairly expensive city, and $10 or $11 isn’t much anyway. As you can read, the average or median (it is not stated which) hourly wage for a “low wage worker” before the first stair step was $9.96, so it’s not as if the new floor was much above the equilibrium.
Another issue is that the study depends on the researchers comparing the real Seattle to a “synthetic Seattle” based on nearby suburban Seattle zip codes that had comparable characteristics. My brain is a bit frozen at the minute, but if I let it unfreeze and gave it some thought, I could come up with some methodological problems with that.
Or, maybe we’ll find out that this research was purely political pseudo-science meant to peddle an ulterior agenda.