plz someone can you say me WHY !!

Uhm, you forget to mention how you killed him, since that probably is the reason why you got no exp.
I think you probably disintegrated him, either with the disintegrate spell or with the chromatic orb spell.
If a monster gets killed by disintegrate, you get no exp and no loot I fear.

The best way to kill him is to cast magic resistance on him (yes, this actually lowers his magic resistance) and then cast harm on him.

hummm...I have killed him like that:
-12 Yoshimo trap around him
-magic resistance cast on him
-cast harm on him
and *BOUM* when he became enemy all the trap explode it his big ass!
he die without i touch him with a weapon

I killed Firkragg whit two swordman (minsk and valygard)+ i also had two man whit a bow
(my own charecter and imoen). I didint use any magic on him or my charecters,
i just hack and slashed him. Beat that guys.

I surronded him with Earth Elementals and sat bacl and Realaxed. Ahh

check out a post done a few weeks back.
some guy (can't remember his name) killed him with one hit with his monk. he posted a picture on the web as well. very cool!!
i killed him by distracting him from one side with a magic protectered fighter and from the other just hacked him up with my ranger and thief.

I actually killed Firkragg too, I got XP but no Loot....hmm and I just gased his ass.

Haha

You've answered your own question!!

Traps are effective, but, like summoned creatures, they don't give you exp. when you have "killed" the enemy.

You'll have to hit it with a melee character or an offensive spell.

IN other words, you character has to deal at leadt some damage before the game registers that you have killed the enemy.