Till now even I did not know this fact unless and until you posted it here that we do not need actual marketing knowledge when it comes to online marketing.
I do agree with the counselor above, that SEO is technical, whereas Marketing is not.
Thus interviewers find you to be over qualified, i.e, the MBA degree or under qualified, i.e, your knowledge regarding online marketing process.
Here I would request you, if you are planning for jobs in Digital marketing, enhance your knowledge regarding software and animation, coral draw etc.
Online marketing is communicating with the cli