Novel slavery...

Probably because the author was and is human. Normally u wouldn't like to place your own kind into slave even only inside a story.

Though novelly my answer is human greed and adaptability was higher than the other race. Probably.

Well, usually in a story other races became a slave inside human kingdom and humans became a slave inside other races kingdom. 

This is the correct answer.  There is no point to trying to figure out the logic of the scenario because the reason it exists is so that the protagonist can have a cute elf slave girl.  Any circumstance that changes the scenario is unacceptable since it would get rid of the lave waifu. 

The reasons as to why other races are enslaved are the same as to why human are considered the dominant species:

1. Human Recovery: Unlike long living races (Elves,...) human live shorter lives, which causes them to reproduce more often then Elves. Given the sheer quantity of humans, loosing 1000 human soldiers to kill a single elven warrior will hurt the elves more in the long run, especially because training those soldiers can be done in a few months/years, while said elven warrior took decades to get to his skill.
Since humans can recover their losses in manpower more quickly than elves, they tend to win drawn out conflicts simply by whittling away their opponents forces.

2. Human Adaptability: Unlike dwarves and other races that are specialized towards one type of environment (underground, ocean, etc.), humans can prosper nearly anywhere. More variety of space leads to more inventions to tackle problems, allowing humans to even prove a challenge for other races in their specialized environment.

3. Humans Drive: Unlike truly powerfull races (Giants, Dragons, ...), humans are driven for improvement and survival. Said strong races however have a problem: To improve, they'd either have to challenge equally strong foes (thus cutting down their numbers), destroy everything around them (and starving to death) or simply sleep away the time to allow new food to come, thus limiting their achievements.
Dragons still are considered fearsome apex creatures, but their limited activity makes it less likely to rule the whole world, whereas humans can thrive even in the most desolate places, growing their strength and pushing towards better places.

4. Human Intelligence: Any race that falls behind in an arms race will lose sooner or later

5. Humans Control of their Instincts: While still driven by instincts, human tend to be a lot less influenced by them than Orcs, Trolls, etc. allowing them to take different approaches to a problem instead of falling into a fight, flight or f*ck mindset.

6. Human Base Stats: While other races (depending on author gnomes, kobolds, goblins, ...) fulfill many of the above criteria, their base stats tend to be lower than human, making their starting point even harder than for their human equivalent.

As a school allegory:
- Dragons know it all, never learn, sleep all day, ace all tests, and maybe fail the finals
- Elves ace all tests and the finals, but repeat each year ten times to do so.
- Dwarves ace their blacksmith and mining exams but fail all others
- Giants get by on a PE scholarship
- Orcs get thrown out of school for beating up the teacher
- Goblins get thrown out of school for trying to r*pe the teacher
- Humans study everyday, work their a** off and pass the final only to continue improving until they either die or become successful
- Lizardmen usually got lost in the swam and never made it to school
- ... 

Thanks for the suggestion  

this become funny thread
lets play aling
again


human superiority on most noivel actually just part of author not wanna create balanced world building or it is intended to be like that

breed superiority?
give goblin better intellect then let see  who will win on number

technology? give dwarf more passion toward industry let see who can make more devastating nuke

adaptation? give beastman more desire to explore the world and let see who become more adaptive

strategy, scheme, and art of war.. give demon race better unison while keep them kinda dark and love create war on multiple fronts.. let see who more devious on tricking other
or just let the elves actually the hidden boss~ oh yes they can, powerful magic, long live, charming, understand nature... fumu sound like advance civilization on disguise for me

there plenty of way to kick human superiority imo human is just one of the race exist on some world

if every race have similar intelect and desire then it is enuf to create check and balance thing 

I got no idea, perhaps, because that is how most novel writers think how things will be in an isekai  

Or in the case of the skaven the ultimate and almost perfect race just slightly below dogs. The fact that they will backstab anyone at anytime for any reason~ 

One Germany did not lose to the Russian manpower look at China vs Japan. The ratios are far far higher. No Germany lost to the physical distances in Russia their supply lines were to long and the partisans did to much damage to the Infrastructure to allow for the continued advance.

On the gun thing it’s also because so many research pathways e.g. dum dum rounds and laser weaponry are not allowed to be deployed in battle so they aren’t researched so there really isn’t a point to get better m4’s when it’s more viable to get better jets and tanks and submarines which is why there is a major lack of surface ship development especially outside of aircraft carriers and destroyers.
Skaven skitter-scurry! Clearly from this the Skaven are the greatest and strongest race!