A while back, youtube allowed really long video times. As time went by, it kept getting reduced. Recently, it was down to a 10 minute limit in 2010. However, they upgraded it to 15 minutes. For anyone that posted when the time limits were greater, their posts will still be there. Currently, if anyone needs to post a video longer than the 15 minute limit, they have to split it up into parts. In September, 2011, youtube allowed users to be able to raise the 15 minute rule to 12 hours. However, users had to provide personal information like a credit card and cell phone number to verify your identity.
Also, Most youtube videos are scraped from websites and most websites just have short clips related to a topic being discussed. The majority of the time, small clips are the only portion of a video that is applicable to a topic. Many website publishers look for video clips related top their discussions, which in turn is short. They find it on youtube to paste on their pages. Usually, whenever you see a long youtube video, it is not related to a topic. For instance, Family Matters season 2 episode 8. Obviously, that will be a long clip on youtube. However, if the topic was discussing how clumsy steve was, it would be a short clip. Anyone else discussing a similar topic will most likely use the same video. Other than that, longer ones are usually made of power points for commercial applications, training, or custom made videos produced by the owner. In some cases, some people place a movie on youtube. With the pace of web posting these days, most don't have time for long clips anyways. But as mentioned above, the limits are in place.