Because it is mainly based on Continental Philosophy, and if you don’t know something about Philosophy normally it is very easy to get lost. Another thing is that it has been taken up by English teachers who are not philosophers themselves and thus they get a lot of it wrong, which is unfortunate. Continental Philosophers use Literature as a basic testing ground for their theories as well as PsychoAnalysis, History, Politics, Society and Culture in general (See Zizek as the best known recent example). Basically, if you don’t know Plato, Aristotle, Descartes, Leibniz, Spinoza, Kant, Hegel, Husserl, Heidegger, Lacan, Derrida, Merleau-Ponty, Sartre, Baudrillard, Deleuze, Bataille, etc you are going to be lost. The works of these philosophers are themselves very difficult, and then when you try to apply one of them to something in literature you are going to get something very arcane, obscure and esoteric. But many times the works being used to leverage as theory are not well understood by the English graduate students and even teaching academics who have gotten their Ph.D.s and so this results in a lot of confusion for English students who are asked to master two disciplines at least in order to pursue their love of literature if they are going to understand Literary Theory. In America and England the number of students studying Continental Philosophy is small but growing while Analytical Philosophy is fading. Analytical Philosophy is only really interested in its own specialized internal arguments. So there are not a lot of academics in Philosophy helping, however many of those who exist are very good. I would like to mention John Sallis and especially his book Being and Logos.
This is as good a place to start as any. Sallis is a fine scholar who has learned to read Plato’s dialogues as if they were theater, where the setting and characters are as important as the philosophy. And from that we find that Plato is continually undercutting his own message. Plato is not just ironic, his is indicating what he believes by a kind of dissimulation of his own positions. For instance, we are continually told that Socrates is a real philosopher and his enemies the sophists are not real philosophers. The main difference is that the Sophists are foreigners, and they take money to teach what from Socrates’ point of view something they do not know themselves but think they know. But as we look deeper and deeper into it it becomes harder and harder to tell the difference between Socrates and the Sophists that he berates. Here in the work of Sallis we get the intersection between Literature and Philosophy with respect to Plato’s dialogues. And we can think of this intersection growing over time instead of specializing to the point of inanity which is what seems to have happened in literature and philosophy both, but which is not endemic to either approach to writings from either the past or present. If you take Sallis as a model and the way that Plato himself blends narrative, philosophy and mythology then we can see that it is not necessary to separate them out from each other and in fact they can inform each other to give us access to deeper meanings both in literature and philosophy.