Because of this, straight from the California DMV website:
All personalized plate orders are manually reviewed prior to the plate order being made. California Code of Regulations, Title 13, Article 3.1, Special Interest License Plates provides that the department has the right to refuse any combination of letters and/or letters and numbers that may carry connotations offensive to good taste and decency, or which would be misleading or in conflict with any license plate series now issued. Substituting numbers for letters or vice versa may be confused with an existing plate and is not permitted.