The difference is completely and inherently political.
I know the Icelandic government spends some money for this and it shows. This tiny language has way more support then other way more spoken languages. If the Norwegian government wanted I bet the Sámi languages could have just as good of a support as Icelandic. Or if the Greenlandic government had more funds available I bet we would see Kalaallisut in more places online.
Serbian government would certainly support Serbian language voice recognition and synthesis, but probably not with as much money as Iceland would.
The idea that there is a difference between these two things is one of the more pernicious ones of the last hundred years.
Money is power. The exercise of power is politics. They can't be separated.
It certainly sounds like this is a political situation to me, almost to a tautology. The fact that these decisions was made on the basis of financial gain doesn't make them any less political.
The reason why there isn't a huge consumer market for indigenous languages is because they're overwhelmingly systematically unsupported by their respective governments in favor of the non-indigenous colonial languages.
To be clear, that's not Mozilla's fault, and not something they or other random organizations can fix, but as human beings we should all be happy and give credit to those organizations that do their small part.
Yet I have never seen an application that had a Sorbian translation, for one simple reason: Every Sorb also speaks German, so there is no financial incentive to invest in a Sorbian translation. The only things with Sorbian translations are those produced in the local area, e.g. the websites of local governments or local businesses.