For alien motivations, how about they see our development as a future threat, and invade to try and destroy us before we develop the technology that would let us compete with them? If you want to give humanity a break, use Harry Turtledove's idea that between their recon and their assault, we developed faster than they expected and have higher tech than their forces expect... not as high as the aliens, but perhaps they were only expecting muzzle-loading firearms, or no aircraft, or no electronics...
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