A few thoughts about that :
* one major point is that the aliens at a minmum want to have a bipshere that's at least in a decent shape - henc nop major nuking or crashing a dino-killer comet or asteroïd into Earth.
* as far as motivation go, onenovel I read had a race of robotic sentiences evolved from some old probe which had a 'creed' bas on some old garbled lines of code form their remote ancestors who basicaly ordered them to 'find uses for things'. Which meant they experminented with slave labor and using human parts int themselves to see how they could use humans... Something along that line might work.
Another option to explain for a relative lack of ressource could be that said aliens are basically underfunded criminals who think the humans would make perfect novelty slave and their planet would offer plenty of exotic pets and foodustuffs to sell under the counter. The spacegoing equivalent of somali pirates mixed with sudanese horse-mounted raiders. They first gatheed amples to evaluate the market (UFO sigthings, abductions and cow mutilations) and now the don has decided it's time to take charge and really start the market.
Using dumb and fast-breeding goons would be both a way to save on the transport bil and help with keeping teh operation out of the glactic law enforcement's eye. Get a few gigatons of old surplus weaponry scheduled for recycling, a handfull of breeders and move all that into the solar systems would be easier than assembling an invasion force in a settled system. No nosy neighbours to call he cops.
Note : and since these gusy are basically criminals using dumb muscle, it offers a nice explanation for suboptimal planning and tactics : their weapon mix is what they can scroune from the recycling yard and they have little military experience, giving the locals a chance to fend off the thugs...
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