Fridge Brilliance: Why is it the old Lancers are inaccurate and slow firing while the new one is accurate, faster firing, and chews through ammo quicker? It's hard to see how one grew into the other until realizing the retro lancer was an offensive weapon, used for constantly redeploying closer to the enemy and charging forward. Whereas with the modern Lancer they've fallen back to Jacinto and needed something that could function while holding a single location.
Wait what? The new one is the better weapon. Isn't the better explanation that after working on them for a few years they were able to make them more accurate and fire faster, to kill quicker?
The new lancer is actually slower firing and more accurate. Baird said the retro Lancer could dislocate a shoulder, though, so maybe retro lancers were just impractical.
The Locust Horde is by no means primitive technologically, instead using Organic Technology as counterparts for COG developments. The COG use more familiar machinery, instead running on the gasoline-like Imulsion. However, this discrepancy in technological development makes sense lore-wise as well. Humans harness Imulsion, which is actually what's killing the planet, without knowing the truth behind it. The Locust, developing underground and living next to the stuff, would have discovered its danger long ago, prompting their development of biotechnology (which would never need Imulsion).
Some more Fridge Brilliance: Why is it that the locust drones are smaller and weaker than the females of their species, the Berserkers? In reptiles, the females are always bigger than the males, and Locust are a species of underground-dwelling lizard men.