No mandate. However chromebooks when they came out were a HUGE improvement over low end windows laptops. They are crazy durable, include instructions for when you pour soda into the keyboard, are durable enough to be thrown, leave a mark on the ceiling, and still survive. They run well even on old hardware, 4GB ram is plenty. They also don't get viruses, don't require any local expertise, don't require backups, don't require any software licenses, no virus scanning, and have minimal requirements from central IT. If a student manages to kill one you can literally just hand them another and say login.
So the savings in hardware costs, reduced support costs, and the "it just works" design is pretty attractive to school districts. Who'd have thought a $180 laptop could be durable, useful, no crapware/malware/ransomware, and last a decade?
Sure it would be nice if it didn't depend on google, but nobody else was stepping up.
My kid has had three over the last decade, the last we paid $50 to keep when she graduated. It's still being used years later.