But I find it very bizarre. I can't name any store: pharmacy, grocery store, discount dept store that does not stock them openly on a shelf.
There's a simple reason for it: Stores only stock items that sell frequently and turn a decent profit. They don't stock stuff that sits on the shelf, takes up lots of space and generates no revenue.
Diapers, unfortunately, are exactly that. People who truely need them, will get a prescription for them from their doctor. That's the only way you can get them for free, and why pay for diapers out of your own pocket when you pay for medical insurance / taxes for the national health system and can get them for free?!
In my country, grocery stores and drug stores do not and cannot legally have a license to handle prescriptions. So the only place where you can turn your prescription into the medicine or diapers you need is a pharmacy or - for items other than medicine - medical supply stores. They have a license / agreement to handle prescriptions and get the money from the insurance companies.
I'm not sure where incontinent people get their diapers from in the UK, but evidently they don't pick them up in grocery stores but get them delivered directly from the National Health Service (NHS) or pick them up at some NHS place.
You may find that bizarre, but your medical health system in Canada may work in a different way...or Americans have no health system at all and may as well pick up the diapers with the rest of the grocery shopping!
Peachy