Why cant evolution be said to progress?

There is no real ‘progress’ in the idea of evolution. Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection. The only progressive trend in evolution seems to be that more and more complex body designs have emerged over time. But this does not mean that the older designs are inefficient. So many of the older and simpler designs still survive. Bacteria, one of the simplest life forms, inhabit the most inhospitable habitats like hot springs, deep-sea thermal vents and the ice in Antarctica. These habitats cannot be inhabited by the recently developed species. 

The newly generated species are not in any way better than the older one. It is just that natural selection and genetic drift have together led to the formation of a population that cannot reproduce with the original one. (So human beings are not the pinnacle of evolution, but simply yet another species in the broad spectrum of evolving life.)