Cavities are known to be hard to deal with. Dentists say that Teeth at the gum line bend the most under pressure. This continued bending at the gum line is one of the reasons why depressions like these structures can form. Sometimes it also depends on your dentist.
However, sometimes the filling is also bigger than the cavity. You may not have solid teeth around the filling to help biting, and this adds additional weight to the filling, causing it to break out. You might grip on the filling, which will make the filling loosen up sometimes. Regularly in this circumstance, some dentists would prescribe a Crown because of their quality.
Another reason why they would prefer a crown is for its durability. When the filling isn’t attached to the tooth, then there would be some pressure along with the bite in that area. In case more weight is added on the tooth then there are more chances of the tooth filling falling out. Sometimes there is less lacquer applied, which is not enough to hold the filling in place.