Why has the drug temozolomide replaced the combination of procarbazine, carmustine and vincristine?

Newer treatment. In treatment of Cancer, newer drugs come along and are then compared to the standard(old/conventional treatments). If the new treatment if better(more effective or less toxic) then the new drug takes over and the older drugs put on the back burner. That is what has happened in this instance. This is what has happened in the treatment of brain tumors.