I suspect because it fits with the modern narrative of antistructuralism; much that is described as "Gnostic" these days seems to be more a reaction against modern orthodoxy than the proto-orthodoxy of the ancients, such as it existed at all. The mystic character of much of the corpus makes it all the easier to read a modern progressivist narrative into it. As an anthropologist, I find such recolonization of the past unsurprising and unalarming. Those facing the seemingly all-powerful inertia of a state-sponsored civil religion often find solace in seeking the company of their ancient kindred, however they understand them, and looked at from the vantage of the persecution years, it's even fair. Modern Gnosticism may not be a carbon copy of 2nd c. Gnosticism. But I suspect the beleaguered Gnostics of the 4th century onward could have much sympathetic conversation about the perils of minority faith.