The well-known adage 'show, not tell', is constantly bandied on creative writing courses and in articles on writing fiction. I fully agree this is very sound advice. It is therefore most frustrating when you read highly successfully authors who 'tell, not show'. So why do they do this? Probably because it is far easier, and amidst their signing tours and talks, it becomes increasingly more difficult to actually find the time to write. My article on this subject, Show not tell—for starters! proves the point by taking a couple of examples from a very well-established crime writer.
The fact is that many famous writers get away with 'telling', and this can be very misleading to wannabe novelists. (Telling was the traditional way to tell stories, in any case.) The reason why you should take the 'show, not tell' advice very seriously as an author is that publishers demand it of a debut novelist: because it shows they have real writing ability. Once you start 'telling' the reader stuff then you come to the forefront as the author and it tends to ruin what publishers call the 'suspension of disbelief'. If you show the same information through the eyes or mind of a character, however, your authorial presence remains remote and the reader can become more deeply involved in the story. Follow the old adage and your reader will be happier - as will your potentional publishers.
Check out my article for a more detailed treatise, with examples.