My guess on 'intermediate' would be due to:
1) Size. To us they are manageable and still considered smaller than some other boas, burms, or retics.. but to the average person just getting into the hobby, or learning about snakes at all, a dumerils can be intimidating. They're still on the middle-to-larger end of the scale as far as many species most commonly kept in the hobby (e.g. Balls, corns, kingsakes, even some carpets that get long, but are not terribly heavy bodied)
2) some can be difficult to get started (I have also experienced this first hand)... possibly due to them being so shy, and not really liking an audience when eating. Our girl eats great, but she's also 9 years old, and gets fed a large/jumbo rat once every month to month and a half, otherwise she starts getting fat rolls.