I think, it's more "If to stop" question, rather than "why continue". I've had the same doubts not a long ago (actually, when introduced my comic here, on Tapastic) and there's the answer I gave myself -
Ok, Lizard, you faced the horrible truth - your comic, probably, isn't great. People see it and walk away. And it's time to consider, what is your comic to you. Is it just a Hobby? Are you doing it for your own amusement, and it's no different from a papercrafts or glueing models? But if you concerned with readership, it's probably not just to kill some time. So, Is it a business? You hope to have some kind of income from cartooning, so you invest into your comic - time, knowledge, actual money (domain, server space, promotion, maybe printing costs) and you want to get something out of your investment. And when you see that you don't have what it takes to go (semi)professional - possibly you should stop. Unless - you consider your comic as a School. Your comic might become your very own Center for Cartoon Studies (or Kubert school - depends what kind of comics you are into). You still investing time, money and effort. But the knowledge and experience is what you get in return. And then you should stop only if you don't wanna learn.
I still want to learn a new things, and I like my character and I believe in it's potential - so I choose to carry on. =)