Bitstream Charter is recommended by typographs as the best open source font for writing and its license permits its redistribution (cf. http://en.wikipedia.org/wiki/Bitstream_Charter) :
http://practicaltypography.com/bad-fonts.html
http://practicaltypography.com/charter.html
Fira Sans could also be bundled.
I’m not sure this is a question that can be answered here. The answer would be something like “because the developers chose another font”.