Charity Majors:

The mandate is one way of putting organizational muscle behind a decision. It’s a funding mechanism. It works by acknowledging that “hey, we are all going to be a little slower for a bit while we figure this out, and it will be annoying and expensive and we accept that.”

Without funding and a mandate, you’re effectively telling your employees to build these skills in their spare time, if they can and if they feel like it. Which is the same as telling them “this is NOT a priority, we are NOT willing to fund it.”