Not sure the reasoning, or even whether it was their decision-- I know they didn't have a lot of sports the Metro sponsored, and I think there was an expectation that they would make sure their program could consistently field varsity teams in most of the conference's sports.
They have actually helped form a new conference in the Richmond area, called (interestingly) the Virginia Metro Athletic Conference (VMAC, pronounced vee-mac) with Grove Christian, Guardian Christian, and Grace Christian (Mechanicsville).
The Metro, meanwhile, has added new VISAA member Gateway Christian in Virginia Beach to maintain their old eight-member structure.
