I'm guessing because they don't want to. Their "mission" is much larger than Firefox.
It’s like saying, yeah I solved cancer, but my vision is so much bigger than that! I also invented an Excel formula that allows shoe stores to advertise 2% less expensively to dogs.
We really need an alternative browser. We have one. But the people running it want to use your donations to do other things that are 100x less important.