1) chromebooks are relatively inexpensive - no barrier to entry
2) you get a full fledged browser - most normie use cases are covered
3) you get a linux VM and linux apps - developers and ethusiasts will be happy
4) you get access to android apps
5) mobile OS so good battery life
6) mobile OS so quick boot
7) mobile OS so low compute demand
8) many have a touch screen - not available on Mac OS and garbage on Windows
9) you can literally powerwash and restore from the cloud in no time
10) schools have been using them for years - an entire generation will be familiar with it when they enter the workforce.
The list goes on an on. It might not cover every use case but it's ready for mass adoption. If they ever start coming with dedicated graphics is game over for their competitors.