It's because they didn't post the videos. The fight club videos were from that ridiculous blog The Conservative Tree House. It's not even him in the video, you can plainly see that the kid in the video isn't 5 feet 11 inches tall.
my opinion only
As far as I recall, they did initially post the video to the stories (and no I did not think to take a screenshot, but not sure it would matter in this case), and they were then removed and replaced with the stock content once the video was removed from the original hosting service on the Internet.
Note the subject of the video does not actually match the Title of the article. Both title and video embedded are about the case, but typically, editorial guidelines at most publications for Online Media would require that if you are talking about a subject, in this case a video, (especially one which the situation is "Is this or isn't this?" such to the extent you have to ask the question from the family, thus not readily apparent), that you would include the video, or specifically state why you are not (Graphic content, Trademark Infringement, etc.) Otherwise, your piece is as relevant as a piece titled "Beautiful Sunset over Galveston Bay witnessed last night" and then instead of a video or static photo of the Beautiful Sunset referred to in your article, you just post a picture of a Boat in the Bay, in the middle of the daytime (no sunset). Any reader would then say "Why did you tell me about a beautiful sunset and then not show me? I would like to see it, especially if I would think it is beautiful as well."
Coincidentally, the video was not an issue for the Internet Host from July 2011 when it was originally posted to the very day (almost an entire year afterwards) that the research indicating the irrefutable links to the original video were posted by a few blogs, one of which is considered a "major" blog. Could this be just coincidence? Could be, but personally, I doubt it.
As a result of that information that was published, the Palm Beach Post and WFTV responded to the information with the linked article. I assume that they believe there was at least compelling evidence enough to ask the question, because if it was obvious that it was not him, it would be like a reporter asking a man walking his German Shepard, if it was a Boston Terrier. Why did they do this? IMO, it is because they know that it will likely come into evidence, and they wanted to get ahead of it.
As for your statement about his height being 5'11", I have seen this mentioned many times and perhaps best discussed on another thread, but I thought the same thing, especially after seeing the 7-11 Video and some of the pictures posted of him. I was thinking that either the ME was wrong, or that people in Sanford, Fl appear to be abnormally short in stature compared to him.
Upon researching, I believe (IMO) the reason why his family initially claimed he was 6'2-6'3 and the ME claimed he was 71 inches (5'11), is because by the time the ME took the measurement, full rigor mortis had occurred, which makes it impossible to completely straighten the spine to get the same measurement as if one was alive and standing up. I would suggest the family's statement is more accurate as a result, but the easy way to confirm this, and they will likely come into evidence at some point I would guess, would be to obtain Trayvon's medical records. He played Football, and unless they do something strange in Florida, all programs I know of, be they "Pewee", JFL, or High School, all require players to have an annual physical and be medically cleared, so there should then obviously be a height measurement on his chart I would think.