For example a winter tyre that is exceptionally small can be very difficult to obtain. Or something that has a larger diameter than tyres typically do. Or the tyre has a small diameter but is very wide. Or has some other required features but also needs to be protected against punctures?
At least getting winter tyres for my child's bike was absolutely impossible in any bike shop I visited, because even the ones selling relatively small winter tyres had them with a diameter two inches bigger than what fits that bike.