The Soviet Buran program decided to use a carrier aircraft to move their orbiter and Energia tanks similarly to the American Shuttle Carrier Aircraft. However, the Soviet Union built two types to do the job.
The famous An-225 project was started early, but they also decided to build a backup aircraft based on a Myasishchev 3M bomber. This aircraft, the VM-T Atlant, was finished much earlier. Since the An-225 wasn't ready yet the Atlant played delivery truck for the initial and only Buran launch.
Of course the An-225 is much more capable than the Atlant, but if the Atlant can do the job why go through the effort and expense of developing a second massive airplane?