Fairy Tail is pretty cool

[Brian]

Shonen manga are primarily marketed to high-school aged and younger boys, that's why.

Read seinin.

[Dracos]

Merc:

Bad writing.

But contextually there is at least that they're constantly running into things that are challenging for the main fairytail crew, who started way more powerful than Lucy and got more powerful too.  Her power set has gotten stronger, but more relevantly its gotten more eclectic.  It's like:
Natsu: Eat stronger fire, improve attack.
Ezra: Gain new armor/more magic, improve attack.
Gray: Gain new insight, improve attack.
Lucy: Gain new key, get new single summon.

But it's bad writing that basically every time Lucy is set up to kick ass, it's pulled out from under her.  The generic "Non-dragonslayers need  not apply" rule going on here will ensure she still doesn't get to do so.  It's a shame since the setup of: Okay, you've had these things, but now you finally got to train with them and master a good attack spell (Which frankly, she lacked hard), even if it got nerfed lamely in arena encounter it should've shown up here.  She's also oddly not taking all tweleve keys.

Edit:

And yes, it's also that its a shonen manga and showing her getting her clothes knocked off is fanservice.