The non-flippant answer that is related to the flippant answer: because there are dozens of distros and within those distros about a dozen browsers if you factor in all of the Mozilla forks. They could probably choose a blessed one, but any individual blessed one turns out to have an incredibly small user base (Even Ubuntu, to the extent that numbers can be generated as a secondary signal, looks to be about 1% desktop OS share).
The non-flippant answer that is related to the flippant answer: because there are dozens of distros and within those distros about a dozen browsers if you factor in all of the Mozilla forks. They could probably choose a blessed one, but any individual blessed one turns out to have an incredibly small user base (Even Ubuntu, to the extent that numbers can be generated as a secondary signal, looks to be about 1% desktop OS share).