Watercress is very perishable and needs to be kept moist. I believe it was implicated in an outbreak of some disease or other. I can imagine stores can’t be bothered to take a risk on a low profit item.

Also from Wikipedia…

Concerns

Watercress crops grown in the presence of manure can be an environment for parasites such as the liver fluke, Fasciola hepatica.[14] By inhibiting the cytochrome P450 enzyme CYP2E1, compounds in watercress may alter drug metabolism in individuals on certain medications such as chlorzoxazone.[15]