Because Mozilla and its execs were asleep at the wheel while Google started to eat its share. This is what you get when you stop competing.
The execs are still competing, they're winning, and getting exactly what they aim for, personal wealth.
I mean... it was probably that too, but Google abusing its search monopoly to market Chrome probably played a major role as well. Or if you like - Mozilla's mistakes are such that Google probably could have won fair, but that's not what actually happened.