I can think of several factors why the AT might be considered more dangerous, even if it were not more dangerous in actual fact..

The AT is a very long trail. In a few specific places there have been criminal activities that were highly publicized, including a few murders. The publicity was much greater than for most murders (think of the dozens up to hundreds of murders each year in New York, Chicago, DC, Philly) in part because the murder of a hiker on a trail is surprising and therefore more "newsworthy". Next, the authorities are more likely to use publicity for this sort of crime in an effort to turn up clues.

Also, because the AT is a famous trail, and a popular trail, any criminal news that happens there is likely to be remembered to have happened on the AT, as opposed to a little known trail, where the fact of "where" is soon recalled merely as "on a trail somewhere".

Lastly, the sheer numbers of hikers who hike the AT compared to other trails is more likely to lead to a larger number criminal acts on the AT than on other trails, just following the law of averages.

Perception is a large part of how people think and perception is often distorted by emotion or circumstances. The real measure of danger would be crimes per hiker mile, but humans are not built to think in those terms. They must be taught to.