Some element of practicality must enter into it otherwise the heritage movement will be unable to fund any further work, such as surviving in an increasingly difficult economic situation.
They've got more stock than they can handle at the moment - their 4VEP lives off-site, and they don't have the funds to restore the more significant carriages they do own.
According to Wikipedia, of the 5 others the 5BEL trust have 2 of them, the VSOE have stored 2 of them (at least one at Barrow Hill) and one was destroyed in a fire.
If this car was in better condition, then it might have been of more interest to the 5BEL trust as this would have been the 4th of the 5 cars of Unit 2051. VSOE hold the last car of this unit, so they could have had a completely singular train.