Brainiac V is hiding from the other Aliens, Machiavellian humans and his own people.
He's from the future.
His presence on Earth, or it becoming common knowledge that time travel is possible, could create a paradox, or just honestly his brain is a treasure trove of very dangerous information that President Baker, Lex Luthor or Darkseid should not get a hold of.
Brainiac knows how long President Baker stays in office, and what ousts him, if he has even a school child's understanding of early 21st century world affairs.
Or maybe in the 21st century no one (local) yet knows what or where Colu is?
It's altogether possible that Colu isn't part of the local collection of known planets, and you may even need 30th century tech to impel that far into deepest space ( in a fraction of a life time)... Or the Coluans could be disinclined to join the Interstellar community, because they are all assholes. Seriously, read the comics, even the "good" Coluans are dicks, who deserve a good kicking.
Drawing the Coluans, if they are absent, into galactic politics "early" would also create a paradox radically changing the 21st and 31st centuries.
(I doubt any of this has occurred to the Supergirl writers, so don't worry, be happy.)