Odds of Getting In
Tank Commander is a much sought after and thus competitive position. Every soldier who gets assigned to an armored platoon is on track to become eligible for the position. A tank crew could number anywhere between 5 and 15, depending on the type of vehicle. Every one of those crew members, unless they find themselves discharged or otherwise disgraced for any reason, will eventually be in a position to apply for Tank Commander.
This means that your average Gunners or Systems Maintainers need to set themselves apart from every other soldier. They need to acquire more certifications, work more positions on a tank, and perform exceptionally in the field under the stress of combat.
In other words, shoot the enemy. A lot. And don't get shot while doing it.