A very basic article, but the statistic given is that Gear-Up landings occur "often as many as six or seven a week" in the United States. That's for piston retracts...it does not differentiate between commercial and private ops.
I've not flown GA for many years, but I remember throwing the checklists out the window and flying with GUMPS or the very long TMPFFIHC from my days in New Zealand. They're certainly handy, especially if you're flying three or four types like many instructors/scenic/et al. pilots do.
However, the transition to a multi-crew environment and getting into turbo-props and turbines adds another element that requires the use of checklists. Of course, we still use flows which are the same as mnemonics; we just back those up.
I agree that there are no excuses for forgetting to put the landing gear down. In saying that, there are certainly circumstances which may lead a crew to miss it. A 3 mile self-check is in my repertoire to avoid become one of the guys we're talking about and I'm certain something similar is used by other pilots.
Training is certainly at issue here. If you have a 350-400 hour pilot teaching a 200 hour pilot how to fly a light twin, there are going to be things that are missed in the training due to a lack of experience. Even if the instructor has 1,500 hours total time, that may all be circuit and training area time with little "real-world" exposure. Pair this up with two low time pilots in a commercial op, and suddenly the holes in the cheese are lining up.
A little more time focused on the airplane rather than on the people sitting at the end of the runway watching you land seems to be in order.