Motivation. There's something about being in a traditional educational setting - even one like a university where your level of presence and attention is up to you - that makes it easier to concentrate and to prioritize your studies over the other things you could be doing with your time.

I think part of that is simply being surrounded socially by other people who are there primarily for the purpose of learning.

That's very difficult for any online course to replicate, because as soon as you turn away from the computer you're out of the academic setting.

For me at least, that's the primary reason I've never been able to finish an online course.