Why vincristine is one of the most used cancer drug?

While it one of. The oldest anti-cancers, it has no marrow suppressive effecs. As a vinca alkaloid, it interferes with the mitotic spindle formation necessary for actively dividing cells. It is used in many pediatric tumors as a cornerstone drug. Leukemias and lymphomas. It does not work very well in most adult solid tumors.
Vincristine is used. To treat a wide array of cancers in children and adults, including hematologic malignancies, central nervous system cancers and a host of solid tumors as well. It impairs the functioning of microtubule assemblies, such as the mitotic spindle, and therefore blocks cell division. Interesting tidbit...It was originally isolated from periwinkle (aka, vinca), hence its designation as a vinca alkaloid.