Ghastra was previously a human with no a human with no talent to speak of. His genetic module or map was weak and had no room for growth.
Hence, he wasn't chosen by any institution. And although the government snatches the 16 year old kids from each family in a stronghold, they still had to pass through the testing process.
Although not much, some people genetic make up are weak, which means they would also be rejected by the institute. If the institute still accepted them despite that fact and still insist on performing the operation of the Xenograft, the genetic map would collapse which would undoubtedly mean death.
Ghastra had been unwilling to let go of such an opportunity and the desperation and ambitions he had finally led him into directly consuming a blood orb.
Turning him into what he was right now. A mutant, but that was better since he preferred to be a king in this desolate land rather than being a mediocre person in the federation.