DH was ill for years, suffering terrible bloating and diarrhoea after every meal. He lost so much weight some friends actually asked if he had cancer 
. GP just kept saying it was IBS. He took medication, cut out foods that were supposed to exacerbate IBS, did everything GP recommended, and it never made an ounce of difference.
Finally, through his own research and process of elimination, he concluded he was intolerant to gluten. He stopped eating it and his health transformed overnight. Almost a year later he's back to a healthy weight and feels well again for the first time in probably a decade.
DH's sister was diagnosed with coeliac disease a couple of months later. She has osteoporosis as a result of suffering for so many years without diagnosis.
I suspect IBS covers such a broad spectrum of symptoms and causes that GPs can't see past it. They immediately jump to that conclusion and ignore the possibility of adverse reactions to gluten.
I also acknowledge that some people see it as a fad diet. I saw on TV some people being interviewed at a gluten-free food fair in London. One bloke who followed a GF diet, when asked what he thought gluten was, scratched his head and said 'I'm not sure exactly, but I know it's some kind of fat'.
I personally can't understand why anyone would willingly limit their diet. We're used to making adjustments for DH at home now, but he feels awkward at friends' houses, like he's putting them to extra trouble and expense. And he's still extremely cautious eating in restaurants. Why would anyone voluntarily inconvenience themselves and others for the hell of it?