The short answer is 'laziness'. People categorise things, ideas, people etc. We categorize ideas as 'scientific', 'religious', 'western', 'materialistic', 'christian', 'mental stuffs', 'pseudo-science', 'Arabs', 'Jews' etc. Analysing and judging individual people or ideas is far more difficult cause there are myriads of them. It is much easier to try to figure out what group they belong and then judge them according the stereotypes associated with that group.
In debates, this fallacy works wonders. When the devil's advocate want to 'show something wrong with the idea', he doesn't opt to lead the lazy crowd to think through the idea. He opt to convince them that 'this is metaphysical' or 'this is religion' , 'this is physicalism' etc. Then the attention shift from the idea they haven't thought through into what they 'know' about religion, metaphysics etc. The results of the red herring is a misjudgement of an idea that actualy doesn't completely fit the category!