Why was the An-225 built when the VM-T Atlant already did the job?
As you can see in the second photo, the VM-T was unable to lift a fully-assembled Buran. The VM-T max payload is 50 tonnes, a Buran weighs on the order of 80 tonnes. 

So for the VM-T flights they had to strip the Buran to get the weight down, including removing the tailplane. 

The An-225 could lift a fully-assembled Buran, but was not ready in time, so they used the VM-T as an interim solution.