Anyone who has attempted to rehome an animal will know that as a rule of thumb, adoption chances for any ball of fluff beyond 3 months old is discouraging, to say the least. Older kittens have a harder time of it than their younger cutsie incarnates, and adults, well adults can only hope someone will give them a chance.
And so any grown-up cat that gets a home, is a lucky cat indeed. Of course, it helps that there are people who are willing to consider grown-ups but they are few in number because most people still believe the myths about kitties.
The biggest myth is that kittens are easier to train and bond with, but it is just that: a myth. (Someday, when I manage to put up Rheilly’s story, you will hopefully appreciate the love and affection even an adult cat that has never known anything but the streets is capable of, if given the right encouragement and environment.)
In conjunction with the general unpopularity of adult cats, is the common perception that they will refuse to play ball, and learn communal living, ie share living space with other cats. While some cats are avowed catists, most kitties will learn and even come to appreciate having a companion. The caveat is how you integrate every furry into your kingdom. (Again I hold the still-to-be-written Rheilly tale as an example)
As the SCAA folks say, it really is not an impossibility. As with anything kitty, a little bit of time and effort goes a long way in accomplishing the trick.