stick it to Google [0]
I think it would have been far, far more impressive to use Quantum (https://wiki.mozilla.org/Quantum) or WebKit (https://webkit.org), with the added benefit that nw.js is already up and going for the Webkit engine (and likely would have sufficed as an Electron replacement that they could drive forward, but I suppose they own Electron now with Github acquired)
For its not Apple anymore and certainly not Mozilla that is the prime competitor nowadays to Microsoft, its Google (With Chromebooks in the schools, and their small business and cloud offerings) and Amazon's Web Services division that are their main competitors now.
While I don't know how all the business logic went into this and ultimately came up with this being the best calculation at hand, I think this would have played better in the media too (especially if they leaned on Mozilla)
[0] I know Chromium != Google, but its pretty hard not to see that most of Chromium's development is downstream from Google and Google employs and/or otherwise has had some direct influence over the core team