Hi I have IBS and an intolerance is not automatically mild or moderate, you are severely misunderstanding that term! A food intolerance involves your gastrointestinal tract rather than your immune system. It can be as severe and life-threatening as anaphylactic shock, and the reason I am nitpicking is that the attitude that it's 'lesser' has actively threatened my life. Intolerance is not a 'step down' from an allergy, that's not what that word means.
I will use myself as an example: I am allergic to bananas and pork, they bother my immune system and so they are allergies; but my intolerances have to do not with my immune system but my gut flora not having the right staff-member to process, say, fructans. My intolerance to alliums is severe and can be life-threatening; my allergy to pork is mild and never is.
I agree with everything else being said--but, and I cannot emphasise this enough, people also used to die of 'intolerances'. Intolerance to a food still causes severe illness, and yet because we have 'intolerance' and not 'allergy', well... I know people with allergies don't even get their life-threatening allergies taken seriously, so you can imagine how bad it is when 'it's not even an allergy you're just intolerant'.
And to the larger conversation I'd like to add: Diarrhoea may just an embarrassing, minor, temporary affliction today in the developed world; but that is recent and only possible because we have access to clean water to drink, toilets indoors, and modern antibacterial chemicals. It was a death sentence for most of human history. It still is if you're part of the developed world and on the street, or in an institution, where you don't have access to those things!
'Why is IBS so prevalent' because like above people said, we have access to more kinds of foods AND people aren't dropping dead from dehydration from diarrhoea all the time! For fuck's sake, cholera and dysentery used to be YEARLY OCCURRENCES, and people died EVERY FUCKING TIME.