Initial note: some feral cats do meow but please read on…
It is believed that cats have learned to meow as a form of communication with their human caretaker in making a demand for food normally.
As feral cats don’t have human caretakers they have not learned to meow and in any case in unmanaged colonies there’ll be nobody to meow to.
That said people also ask: “Why do feral cats meow?”. On the same theme, this must be because feral cats who meow have contact with humans, probably feral cat colony caretakers who are carrying out TNR programs. These cats will sometimes became friendly with their human caregivers who feed them. The feral cats become less feral and sometimes semi-domesticated. They learn to meow to their human caregivers.
Not much more to say. The answer turns on the idea that the cat’s meow is directed at people. Although, there are occasions when feral cats do meow to each other. One such occasion is when a friendly feral cat approaches another with their tail up to signal friendliness. The other does not respond and the tail up cat might then meow to attract its attention. This would be one of the few occasions when feral cats meow.
So, the idea that feral cats never meow is inaccurate. It is just that it appears that the meow is a learned vocalisation from domestic cat to person. Perhaps this vocalisation has entered the feral cat population via domestic cats who came to that population when they were abandoned.