This article explains that for those living to a great degree on welfare and trying to get off (those the $15/hr proponents allege sympathy for) face a devastating blow when their pay exceeds about $12/hr. At around $12/hr, that earned income plus all the entitlement handouts they're entitled to equates to about $38/hr of straight earned income. That means that above $12/hr, they have to push their earned income to over THREE TIMES that _just_ to break even and enjoy the same standard of living! $15/hr is the cliff where they are starting to suffer notable reduction in total income, and just before total income crashes, bottoming out around $18/hr where they lose almost all entitlements.
Ya know, occurs to me that the $15/hr wasn't just picked at random as a nice round number evoking "fairness", it really is the highest minimum wage can be pushed before direct beneficiaries of such legislation would go "HOLY $#!^ WHAT ARE YOU DOING TO MY INCOME YOU STUPID *****?!?!?" duly attacking those Leftists who got it legislated. As others note, it really isn't about minimum wage per se, it's about jacking up higher salaries indexed to that minimum - and $15/hr is as far up as that can be driven. The evil at work here is staggering.