What is the reason for allowing pilots to take Pilot Aptitude Battery Test (PABT) only once?
A few thoughts:


They have no need in getting repeat candidates if the selection pool is  sufficient (which I imagine it is). Starting to accept repeat candidates would also make it harder for first timers, resulting in more work and tests for everybody. People also won't go for the fun of it, but will make their single attempt their best attempt.
I believe they would rather want you to nail it on the first attempt as part of the test. In other words: how do you respond to a new situation, how to you respond to the stress of knowing you have one attempt, etc... It would be effective for their purpose on multiple levels. This is more effective than you returning a second time and learning how to do the test.


While I can't comment on other air forces around the world, I know that German Lufthansa implements the same policy for acceptance.