Why only him and old party know how dangerous the boss monster is. They didn't share monster info to others? What a dick
How exp systems work in this story anyway? The dude lvling with old party together from the start but their lv gap is ridiculously far. Instead help him lvling, they just kick him out.
Care because afraid hurting him .... That's soft way to say to get rid of the pest/burden/trash.
Guess author play safe. I mean, in any revenge story, the one who remove the mc always meet unfortunate event. Like dead, public shame etc or if girl, kidnapped, raped, etc ...
His explanation was he only earn XP when monster fall for his trap. What I interpret was, he earns nothing from damage that isn't caused by his trap (e.g. stabbing monster with the sword given by Usui like he did, for example); whether or not he earns anything at all when monster was hit by his trap and dies by other's hand is unclear, but that should be possible.
So, possible scenario is, his friend probably tried to train a mob into his trap to gives him XP (hence why he can be this high level despite having a job with very restrictive leveling condition) but the gap will naturally grow wider the longer the time and there probably comes a point where the gap becomes too large that doing so isn't a very safe option anymore (his trap kills too slow or cannot kill at all, and probably does so little damage that the XP earn is insufficient).
As to why they don't stay in low level dungeon to level him... this can be explained away with all kind of sound reasons; we'll have to wait and see if author will ever explain it (and will they make a good reason or a flop); but off the top of my head as an example, as No.1 party, the dungeon they participate might be assigned by government or something, or it might not be a good impression for high level party to hog a low level (than themselves) all the time.
As for safety as a reason, I mean, this is not a game despite having level systems and everything. It's perma-death. Or even if not dead outright, I'm not sure the healing skill is enough to regenerate severed limbs; disability might also be on the table. And his friends do explicitly say they want to marry him once they retire. If they keep drag him along, he'll probably die before they retire. It's kinda implied there are other normal jobs he can do for a living that doesn't involve going into dungeon. So, it is a good enough reason to me.