I've actually brought this up at Mario Fan Games Galaxy in the past, as seen here:
There are some interesting responses given too. Some include:
1. Internet games in general are harder, as if the attitude is that those who find and install games on a PC tend to be better at playing them.
2. Fan games don't need to appeal to everyone/families/kids/casual gamers, because they're not being sold.
3. Play testing is lacking in a lot of cases.
4. High difficulty is used as a way to hide poor level design or a lack of effort. Kind of like in old arcade and NES games.
5. Devs design for their own level of skill, damned be anyone else.
6. Fan games are for 'Mario pros', who don't want easy or mindless levels.
7. The levels most play tested are early in the game, so their difficulty is actually harder than that of the rest of the game.
Personally, I'd add a few more:
8. The internet as a whole likes 'flashy' things, especially as far as popular online and free games go. But the most fancy the enemies, bosses, gimmicks, etc, the higher the difficulty will usually be, since complex enemies and bosses have complex attack patterns and flashy gimmicks usually involve massively changing the level mechanics/physics on a regular basis. See, Mushroom Kingdom Fusion. It's interesting to look at, but darn the levels are often hard:
9. Easy level design is often mistaken for 'bad' level design. Easy levels are hence often rejected in favour of complex, multiple set piece featuring marathon levels that feel (to the designer) as if they're 'good' while being too hard for most players.
But I'm hoping to avoid this issue with Mario's Nightmare Quest. That's what's partly taking so long, I'm testing, simplifying levels and gimmicks, etc to make it work better.