This semester, I'm taking my 500-level epistemology class. One of the major debates we're covering is the internalism/externalism debate. The question revolves around whether justification, or whatever one wants to call the state of legitimately holding a belief, is dependent only on things the subject has access to (internalism) or whether it depends on some things that the subject does not have access to (externalism).
One problem with externalism is that a brief glance that the philosophical literature shows that when we talk about legitimate belief, we want to be able to give people sound instruction on the things they ought to believe. We don't really look at justified belief from a purely outside perspective. When Alvin Plantinga--who happens to be an externalist--declares his book Warranted Christian Belief "is about the intellectual or rational acceptability of Christian belief," he is not writing from an abstract, 3rd-person point of view; he wants to be able to render a verdict that will reassure ordinary Christians. Even if they cannot find time to read all 500-pages of his book, one assumes he at least hopes they can rest easy based on a someone else's more concise statement of his thesis.
Yet if we are at the job of telling people how to go about believing, plainly we cannot expect them to act on things they do not have access to. Therefore, the kind of accounts of justification that most people are going to find really useful will be internalist kinds.