Because it challenges long held beliefs about how memory operates which can seem very counterintuitive to people. Most people think their memory works like a camera, fileofax or computer but what's stored are only the most skeletal of details from which your memory of an event is literally recreated by your brain every time you remember it.
Some of her research methods are debatable too. For instance, using deliberately implanted memories as a way of manipulating people to alter their eating patterns:
http://www.newscientist.com/channel/being-human/mg18725... . There is a discussion of this as it relates to fat acceptance at Big Fat Blog:
http://www.bigfatblog.com/archives/001675.php (a rather emotional discussion). The problem with this was the assumption behind the research which amounted to "people are fat because they eat too much". It presumes a moral judgement and is not proven in research.
But for any (legitimate) criticism levied at her I think it has to be acknowledged that Loftus has contribtued enormously to the field.