The only really important reason was marketing. There is already a huge marketing force behind Firefox everywhere in the world (most of it free evangelism by journalists), it being the main rival of the MS/IE monopoly. With shipping Firefox (and mentioning it in our marketing material), we are building on that marketing wave. So many people have heard about Firefox these days that we will alienate the want-to-be-different users if we don’t ship that and ship some GNOME-y thing called Epiphany that they have not heard about. This also reminds me of a dialog with Anna in Stuttgart. She told me that (some of) the test subjects had no problem associating the firefox logo with a web browser: they had already seen it!
The same reasoning may apply to OpenOffice.org vs AbiWord and Gnumeric.
So in short, if you want a distributor to default to some piece of free software instead of another, apart from making sure that your preference is solid and usable, make sure people (not only the distributor) hear about it. Alternatively, go and help improve the most heard-about software in each category, so it will suck less.
PS: I miss the days that I used Epiphany: I am using Firefox now, because that’s what we are shipping and I am supposed to eat our own dog food.