Why did the USSR need to build a new aircraft to move its Buran shuttle?

To move the program's parts and the Buran itself, sea, land, and rail logistics had become obsolete and inefficient at the scale of the Soviet Union. The reduction of ground structures at Baikonur and the start of heavy-lift launcher manufacturing also increased the loads that had to be moved by air. The aircraft that covered that task was the Myasishchev VM-T Atlant, a variant of the Soviet Myasishchev M-4 Molot bomber, which played the same role until the Mriya became available, something that did not happen until after the Buran had already flown. The Atlant could carry loads of 50 t, while the planned projects called for carrying up to 200 t to Baikonur.

Antonov had already built the An-124 Ruslan, then the world's largest cargo aircraft, but even that giant could not carry the Buran on its fuselage. Using it for the air-launch of a space vehicle was not entirely suitable either, because of the vertical stabilizer, which stood in the path of the space vehicle's engines. Antonov could not modify that feature of the An-124, because the aircraft was nearly finished and approved by most of the aviation regulators that granted it certification.

Like its American counterpart, the Buran orbiter traveled from its landing sites to the launch complex on the back of a large jet, the Antonov An-225 Mriya, designed partly for that task and the largest aircraft in the world to fly repeatedly. Despite being the world's largest cargo aircraft, the Mriya was not really cost-effective and was only used when the load was too bulky to fit in an An-124.