How can soft tissue exist in dinosaur fossils?
keratin and collagen are an incredibly tough molecules so if it is isolated from the environment (oxygen and bacteria) it could easily survive for that long. Not all proteins are equal some are very robust other very fragile. We have long known protein and carbohydrates can survive for incredible lengths of time under the right circumstances, amber is famous for it. Rapid deposition of minerals can produce a similar if less powerful result.  

In this case the keratin was encapsulated by water deposited minerals that sealed it away from the outside environment. There are other examples as well, natural mummification can lead to some preservation in the form of biofilms and impressions. But the most dramatic are amber encased specimens. Amber is a quick forming and creates a perfect air seal. 

here is a dinosaur tail preserved in amber.