Simply put, they were afraid of losing their funding.
Sure, they let him go through the motions, go through assessments for compatibility, but when it came right down to it, they passed him up every time for someone else. Anyone else, because Dad had been at the forefront of designing and piloting - and funding - Jaegers and they’d lost him.
He’d been a kid when Dad died, and Mom was already gone (Trespasser took her, and was probably the reason Dad fell head-first into Jaegers), so he’d split the rest of his childhood between living with his grandparents, and staying with Dad’s old friends Alan and Lora during Ranger training.
Alan and Lora were both scientists in the Los Angeles Shatterdome. Lora was part of a team analyzing and studying the Breach in the hopes of closing it, while Alan worked in Jaeger design, looking for better and brighter ways to keep everyone safe until Lora found the secret to closing the portal in the sea.
The first time Sam saw a Jaeger up close - got inside one - was years before he could start training in earnest; he’d helped Alan carry a bunch of equipment into an old, decommissioned Jaeger being retrofitted for training purposes.
It was a stunning machine, even with the scar of damage running from by the Conn-Pod down near the reactor, all pale gray and lines of electric blue. More than once, Sam forgot the point of the outing was for him to help Alan install training software, and a program meant to help buffer, assess, and protect new pilots while they learn, and enter their first Drift inside an operating Conn-Pod.
He couldn’t wait until it was his turn inside the Jaeger, whose old designation had been changed to simply Tron, short for Trace-On (part of the training software).
Meeting Quorra had been pure luck.
He was supposed to have left the Kwoon a long time ago for - joy of joys - a budget expense review; more and more important people were starting to voice their displeasure with the cost of the Jaegers and the Jaeger program, and Sam should have been out there unruffling feathers.
But these punks had dared to badmouth Dad, implying that the best way he’d helped was through his pocketbook, and he’d been delusional to actually go out there and try to fight.
It struck a nerve, not just because Sam himself was already running into obstacles to his attempts to fight.
He didn’t want any more kids to have to grow up without parents because of giant aliens; it was a world problem with a definite solution.
Making a stand on that point was noble, but he maybe shouldn’t have tried to make it with six to one odds.
He was in for it, and he knew it.
Then Quorra intervened.
She was just suddenly there, manifested like a fiery spirit of vengance, broke through the melee with a few well-placed strikes, and pulled him out of there.
He’d resented her for it a little, at first - he could fight his own battles, thank you - but instead of trying to scold or protect him or just generally drive him up the wall, she instead chatted happily with him like they were old friends, and steered him toward the mess for lunch.
By the time lunch was over, they’d talked about what books they’d read recently (Quorra offered to let him borrow one of her Jules Verne books), their favorite board game (Sam offered to teach her how to play Go, because Alan was an overly cautious - i.e. boring - opponent, and nothing would convince Sam that Lora wasn’t cheating, she won all the time), and how far along they were in their training (between Sam’s languishing struggles and Quorra’s meteoric rise through the ranks, surprisingly close).
They were practically inseperable after that. She hung around reading - either more adventure novels, or reading assignments from their training classes - while he was stuck in more and more meetings. She had a standing invitation to join him at Alan’s and Lora’s quarters for dinner. He played with his phone - and played lookout - while she met up with her boyfriend; he was one of the Shatterdome’s security guards, and never said much, but he’d waved like a dork the first time he met Sam, so he was cool in his books. She heckled him good-naturedly about his lack of a love life while they trained together.
She was the sister he’d never had.
So naturally, she was his first, and only, choice for a partner when it came time for them to finally - finally - connect to the Drift.
Inside Tron’s Conn-Pod.
Sam’s hands were almost shaking as he put on the circuitry suit underlayer of the Drivesuit.