Novel slavery...

There is a limit to how often you can pull a new weapon out your ass, sooner or later you will hit a plateau technologically (which is why we are still using M-16s and their M-4 spinoffs). And remember, one person can only use one main weapon at one time. So what if you got a "Bow of Insta-death +200" when there are 3-4 people shooting at you? At best you'll kill one of them before they pincushion you.

Sometimes, there simply is no getting over a numbers disadvantage. Even Germany with all its technological poweress simply got steamrollered by Allied numbers in WWII. Russia demonstrated it even more so. 

it's standard fantasy setting ever since tolkien that humans outnumber all the other major intelligent races: elves, dwarfs, halflings (I think, I need to check), beastkin/lizard-kin, etc.: being balanced (no major weaknesses), adaptable, etc.: so survival of the fittest.

Being balanced, humans are also good at negotiating with other races, and getting more benefits from trades (like getting dwarven weapons), whereas elves' pride means they can't take advantage of these benefits. humanity's balance: means we develop in all areas: magic, metal-work like weapons, all elements, etc. whereas elves only develop magic/nature, and dwarves only only metalwork, halflings mostly develop only rogue skills (and don't work on magic at all), and beastkin/lizard-kin only develop fighting/hunting techniques.

you can think of isekai slavery setting as the point in which humanity's development has overshadowed the the other standard races (except for demon kind with its demon lord) too much that humanity will definitely win against them if war breaks out. many of the more prideful races like elves still cling to their (past) pride, but the wiser individuals already realize it's too late to beat humanity, so even if they know about the enslavement of a some members of their race, they don't actively start wars against humanity.

if you think that the other races are too stupid or something, please remember that the novels we read are written by humans for humans, and that isekai novels with slavery are often supposed to be wish-fulfillment. Also, note: japan is a pretty homogenous country: elves with their pride is like their metaphor for caucasian foreigners, other races is like a metaphor for other types of foreigners (except more beautified for the japanese readers' wish fulfillment), so japanese readers probably identify with humanity more 

It can be driven by religion. Look how the Norse failed in the Greenland colony. For some centuries they thrived, then climate change happened, and failure ensued. Some argue that religion played an important role in their demise. 

if it not explained then either author not care or it not that important for story

as for my opinion for it
meh~
case to case there plenty reason I can give
plainly simple less united, numbers, technology, internal strive and deux fux... GOD WISH IT! 

So what you are saying that if you are born of another race, other than human, you pretty much f**k then?

You cannot grow any longer, other than the limit of your bloodline.

Also in their race entire life did they really did not learn anything at all about progress? anything at all? so they stay in a medieval time like setting because they just forget about the rest of the world? 

Doesnt work when they have a gun point on their forehead. Any species like that would be wiped out before they reached higher sentience. 

We are all slaves to novels...

...oh wait. We're talking about slavery in novel, not about being slaves to novels. Aight, imma head out  Hope you have a great day!  

Breeding faster is only useful if you have the resources to support those numbers. If the races start from a point of balance, then humans should hit the limit of their resources first before getting the #'s to take the other races territory.
the Goblins in Goblin Slayer are winning without a need for that intelligence. But that world has to many threats to properly deal with all of them.