Mirror Cage said:
I wonder if the canon expressly states that the other races are that inferior or if we just assume that the space marines are so much more powerful because they're the ones that the canon tends to focus on...
That really isn't it. The rules themselves are what make a Space Marine so very powerful. To start with, your standard tactical marine's armor will outright deflect 2/3 shots that manage to make contact (with exceptions of course). A terminator ignores 5/6 shots in the same fashion. Even should something penetrate the armor, their toughness is higher than most other troops in the game. Combined this means that it simply takes a lot of firepower (both in terms of weight in fire and quality of fire) to cause a casualty.
If you go from there you find that marines are incredibly skilled with their weapons and, as such, have incredible accuracy. Their standard weapon, the Bolter, is powerful enough that there is little in the game that it is incapable of damaging (though there is a great deal that it is unlikely to damage). To put this in perspective, most other armies rely on specialized weapons to have any hope of destroying certain units.
Finally, you have to understand that Space Marines are an army that is literally capable of fighting to the last man and are, if I remember right at any way, outright impossible to break and difficult to pin.
Simply put, a unit of even basic marines is more effective than even the specialist units of other armies and on a model per model basis, marines are as tough and lethal as the heroes of other armies. Their
one weakness is simply that they are incredibly expensive and as such you will
always be outnumbered by a significant margin. In an even numbers match, few armies could even hope to stand toe to toe against them.
Canonically, they are even more effective. A single company (about 100 marines) is considered enough to pacify a
system of planets. A full chapter (1000 marines) can lay waste to a sector of planets.
