Why do the B747 of NASA Shuttle Carrier Aircrafts (SCAs) need additional vertical stabilizer?
The AN-225 was designed primarily for carrying the Buran, so they didn't have to make structural changes to an existing design to accomodate the carrying of the shuttle. The aircraft was purpose built for that mission (although it never really saw much of that in practice).

The NASA 747 on the other hand, is a stock 747 that they added the ability to carry the Space Shuttle on its back. They couldn't remove the vertical stabilizer on the 747. Due to the shielding effect of the airflow when the shuttle is attached, they needed additional yaw control that required them to add the outer stabilizers. 

The 747's used by NASA were converted passenger aircraft. The first one, N905NA was acquired from American Airlines made in 1974, the second one N911NA was bought from Japan Airlines, built in 1973 and acquired by NASA in 1990 or 1991.