If the tooth has that much filling in it, a crown may be an option. However, it might not be the best choice. In order to place a crown on the tooth, the dentist would need to drill down even more of the natural tooth structure and filling. As a result, you may need to have a post added to the tooth for more support. Your dentist would also need to make sure the roots are healthy enough to support a crown.

Your dentist might not have chosen to place a crown on this tooth because maybe the tooth might not be able to support a crown and/or you wouldn't need just a crown. Placing a filling in the tooth is a more concervative method and sometimes the only choice one is really left with.