Why is the presence of sand or grit unwelcome when eating cockles?

It signifies that the mollusk has not been sufficiently purged of ingested sediment.

The primary reason grit is experienced when eating cockles is a failure in the pre-cooking preparation stage. Grit is sediment, specifically sand or mud, that the mollusk naturally ingests while filtering seawater for sustenance in its natural sandy or muddy seabed environment. If the cockles have not undergone a deliberate purging process—usually involving soaking in salted water—this trapped sediment, referred to as ballast, remains inside the shells. When cooked and consumed, this trapped material results in the unwelcome, gritty texture, indicating the purification step was inadequate for that specific batch of shellfish.