“Why Optometrist (OD) and not Ophthalmologist (MD)?”

Hi everyone, so the optometrist I am shadowing sprung this question on me unexpectedly. Although I never really sat down and constructed an answer, I did give her some solid reasons but she did not seem convinced by my answers. She told me I should research some more between the two and have a better response especially when interviewing. I am truly passionate about optometry and never really even thought about doing ophthalmology but how do I translate that across when answering a question...

“Why Optometrist (OD) and not Ophthalmologist (MD)?”