It isn't illegal to email records under HIPAA. But your doctor probably doesn't have a system set up to securely email records (such things do exist), and their practice has probably adopted privacy policies that don't allow emailing for that reason. Doctors aren't generally compliance experts, and are much more likely to know what the policies of their place of work allow than the distinctions between what HIPAA allies and what their place of employment has adopted as policy based on the particular technology they've decided to adopt and their particular level of risk tolerance and other factors.