Even without discussing the massive difference in backstab multipliers (which is likely only going to apply once to a given creature in a given battle - there is a 5 minute cooldown on backstabs per creature), the two are completely different in how they deliver damage.

The Bronze Sword is a purely Slashing damage weapon. While there aren't a whole lot of enemies that are resistant to Slashing, I also can't immediately think of any that are weak to it, either.

The Abyssal Razor, like all knives, does both Slashing and Piercing damage - in equal amounts before randomization - added together. Typically, the total unmodified output (S+P) of knives is comparable (not equal, but comparable) to swords of equal tech level. Additionally, there are (IIRC) just as many enemies that are resistant to Piercing as there is resistant to Slashing, but a whooooole lot more that are weak to it.

Finally is the speed and stamina factor. The Razor is faster, producing equal or better DPS (one of the other engineers will have to test for specific numbers, I don't have the time to do so currently) and can continue through more combos, in turn generating more time that opponents are forced into stagger.