Perhaps because they were lucky? What they needed was available for millions of years. Note the survivorship bias there. I am sure you already know it, so please excuse me pointing out this obvious thing.
Sure, could be their environment simply didn't change. In which case, a simpler, less adaptable organism may be fitter than a more adaptable one (if the cost of adaptability was higher).
Species go extinct all the time, there are always very unadaptable species around and most of them go extinct pretty soon after they become unadaptable because environments change all the time in the grander scale of things.