They say humans were adaptable creatures.
Presented with an external scenario that changed their behavioral pattern or ways of life, humans were creatures that were able to adapt to the new changes presented to them.
Compared to the other races, humans didn't have anything in particular that stood out for them.
Dwarves were smart, Elves had a high affinity with mana, and orcs had strong constitutions.
...humans had none of that.
They didn't have the dwarves' intelligence, the elves' strong affinity with mana, nor did they have physiques as strong as the orcs.
They were merely a bunch of evolved primates in the eyes of the three races.
...yet, after around 70 years, to the surprise of the three races, humanity had almost caught up with them.