Maybe because it's still in the process of doing so; it just takes a long time. If "Millions of babies continue to die as a result of the mother's birth canal not being big enough for the head" as Dr. Ayala says, then those babies are not going to grow up to reproduce. On the other hand, those babies who don't die and do grow up to reproduce are the ones who will have birth canals which are more likely to be big enough for the heads of the next generation of babies.
If increased brain size has been outstripping increased birth canal size, it's because bigger brains are a more positive survival factor overall than bigger birth canals (even if that means a certain percentage of babies have to die from inadequate birth canals). This is proven by the fact that our bigger brains have allowed us to develop modern medicine and Caesarian sections which are quickly obviating the problem of small birth canals altogether. And that in turn means that evolution will stop selecting for larger birth canals, because genes which result in small birth canals will no longer be culled out.
Evolution always happens in the future. The bones will be found in the future, the answers will be found in the future. With some 3 billion years of life on earth though they still cannot back up their stupid theory. How much time do these folk need? Seems to me they have flunked the test. Anyone who sits on a chair and writes nothing for 150 years cannot be said to have passed the test.