Experience. Letting other species live and get too advanced always ends badly.
This species of alien is extremely long lived and are psychologically very unusual vs most evolved organisms.
From their point of view letting humans live would amount to foolishly ignoring their experience from the last 100 times someone has developed hyperspace technology.
It's always the same
"First they figure out how to get to other stars, they talk a big game about peace and prosperity and cooperation but within a few generations someone somewhere in their species little empire eventually decides that it would be a good idea to strap a blob of grey goo to the hood of a von neuman probe and strap that to a hyper-drive."
"Sometimes it happens quickly, sometimes it takes until the species has some stupid internal war. Sometimes it's some twisted attempt at conquest, sometimes it's misguided curiosity from someone who thinks they can map the universe with self replicators, sometimes one of them creates a badly made AI with too much brains and too little sense but it always happens."
"The last time half a galaxy had to be quarantined, cut off from hyperspace for half a million years(not trivial even for us) sterilized system by system and that was after every living thing there was turned into nanite sludge."
"We like a universe that has more than one species. We like diversity. We like a universe that isn't being turned into a trillion trillion copies of members of your species in some kind of VR masturbation fantasy."
"We want a universe where it's possible that other species will one day arise after you but we know that if we look away for even a few thousand years you'll have wiped out half the galaxy."
"So you have to die. You have the option of giving up all technology beyond what you'd call the 17th century and sticking to your own planet but I can see you're not keen on that. Yes, with your current population that would mean billions of deaths for the first couple of generations."
"To enforce this a troupe of volunteers will be arriving at your planet soon. Why don't they have weapons capable of wiping your very planet from reality? For about the same reason human hunters don't perform deer culls with nuclear weapons. It's unfair, unsporting and would leave the planet unusable for any species after you."
"Their job is to either wipe out humans or to destroy your scientific and industrial base to the point where you'll be unable to maintain your current technological civilization."