There are a number of factors to this:
1. Celiac disease - this is quite a serious condition which has vague symptoms, but is a serious form of gluten sensitivity. There is better recognition of this, and better testing available (it can now be tested for in blood with high reliability - whereas even 10 years ago, it could only be diagnosed by endoscopy [actually 2 endoscopies, but many doctors would be prepared to make a diagnosis on just one]).
2. Increasing fad/pseudoscience dietary advice. There is a large market in fad diets, self-help books, web-sites, etc. There is also a growing market for "food intolerance" testing. There has been a big advance in testing technology and labs can now measure all sorts of stuff; the vast majority, however, of what is tested for in these "food intolerance" tests does not seem to correlate to any kind of disorder or intolerance. Similarly, a lot of the advice given in these books is of debatable scientific quality.
There has been a particularly big push in the "gluten intolerance" camp, but the scientific basis for their claims is dubious, at best.
3. Increasing hypochondria in the population generally. In part, heavily promoted by the fad/pseudoscience pedlars in 2.