As i rmeber it ooxml backers made it intentionally harder to parse the specs than was necessary ,if it was fully open i believe the open source implementation would have been on par. As it is its subtly broken in annoying ways , and with Word being the default - its version wins out and gets to be the only acceptable submission format. If you notice most doc submissions when its not a pdf being requested will specifify MS's version.And by sheer momentum the alts get less traction.