Marblefire only produced 3 levels ever, and I only liked his first one the most. CFox also had a LotM level and most of his levels were meh (he also made most MBA levels).
Some of my MBP levels were noted by me to be very replayable (based on leaderboards/youtube appearances) and fun, even if they aren't LotM worthy. I've had people thinking my levels are great (imo my PQ levels are the best I ever produced) but I never won LotM.
IMO the biggest obstacle is being original and creative which is why most people don't try their hand at level design. Making overly difficult level is easy. Making a level that is fun while being creative (not necessary original, this is probably least looked for atm) means that they have to spend time to learn QuArK/Constructor, playtest a lot, pay great attention to details, and likely to spend many hours on a level and they just can't be bothered to go for all this trouble. The fact that most have school and other games (minecraft most prominent) eats even more time (esp. free time).
Learning Constructor is probably easy for most, but QuArK users probably have more LotM-quality levels than anyone else, which means that people have to spend time learning a far harder program. The lack of tutorials and the fact that you often need to learn things by yourself eats even more of your time.
IMO it comes out to the point of how much free time you have, whether you can be bothered learning level design, whether you can be creative and think up of good levels that are also fun and above all else: good community feedback. If you get none, then why should you make levels? In a mod you make levels, mod is released, people play your levels. People like the mod? they liked your levels. Long story short, they find it better to do it for a mod than by themselves.
"matan, now i get what you meant a few years back when you said that "the level in mbg is beyond me" after the last rampage i noticed things were insane, and now i truly feel that too" - Dushine, 2015.