I don't think it has to be one thing... I think that they are less expensive, and more usable are big things... having a separate keyboard for typing on is definitely a better use case for many over an on-screen keypad that takes up half the screen.
gDocs is probably a big influence as well... as an all around value, I really like the Chromebooks... from a security standpoint, I tend to like them for mom & granda even more... (with uBlock)
They're also easier to centrally manage and are completely user agnostic devices. iPads can be made to be use agnostic devices also, but you have to disable almost all of the built in apps (since iCloud doesn't really accomplish it), and have people sign into some online service each time (e.g. GApps, Office 365, etc).
There's something to be said for a device where you can just grab a new one off of the laptop cart, install nothing, and login as any user with the full experience available.
Windows domain-connected PCs kinda accomplish that, but with a lot more management overhead.
I never tried FireFox Portable ... hmmm, looks like it requires a Windows machine (or a Linux box running Wine)
That might have been interesting if Mozilla had been able to get it to run on Macs.
We have a spare/guest Chromebook and it's been nice to hand it to one of my two daughters when they've cracked the screen. I'm not in IT but I'm just imagining how much easier Chromebooks would make my life. Especially if the management software is even halfway-good.
gDocs is probably a big influence as well... as an all around value, I really like the Chromebooks... from a security standpoint, I tend to like them for mom & granda even more... (with uBlock)