Plane was made to test USSR's "shuttle", so they only ever build one (+ few spare parts).
There was never a need to make more of them because only one Buran "shuttle" ever existed.
After the fall of USSR, plane was scraped for parts for most of the '90s by company that made it (since it didn't have the $$ to retro fit it to international law standards).
Few years went by, and Antonov company (now, growing fast in air-transport market), needed something bigger than AN-124 and since they already have one 225 sitting in scrapyard, they decided to rebuild it to modern standards (since originally it was made for military use), instead of designing/building a new plane.
Here's a good documentary about AN-225 :
PS. Sadly both "Buran" shuttle and "Energia" rocket were destroyed in hangar collapse in 2002 (cause... was poor maintenance)