I discussed this issue with my fisherman, cockles being amongst my favourites shells.
He told me they couldn't be grown easily like mussels. And to harvest them - on an industrial scale - you need to rake the sand, which is neither convenient nor good for the fauna. On the top of it all, cockles will not keep well for long, you're talking one day before being improper for sell.
Finally, as Séan mentioned, there's not a particular taste/awareness for them in Ireland.
The Kid : before you cook the cockles, you have to bath them for a good while, maybe three hours wouldn't be too much, changing the salty water from time to time. And make sure you check ALL of them are alive. The last thing you want is to have a shell full of sand opening during the boil (yes, it happened to me).