This is because most scientists have already decided that evolution over millions of years is 'a fact'. This is called a presupposition - something which is assumed to be true at the outset, because the only alternative is creation. So, when new fossil evidence is considered, the idea that evolution could be wrong is never contemplated. Take for example animals with backbones. The eel-like sea lamprey and hagfish are examples of agnathans and they live in the oceans today.
Lampreys are semi-parasitic and use their mouth to attach to other fish.
So, fish appeared much 'later' in the rocks than many other animals, and this seemed eminently logical to the evolutionists. An extra 125 million years would be more than enough time, they thought. These are dated at some 470 million years. But even though the first fish now appeared some 50 million years earlier, still there was plenty of time for evolution to have worked, they thought. By the late 1990s, fish had been found even 'earlier', from the Early Ordovician system, supposedly around 490 million years ago.