Cambium is considered as a lateral meristem because

Lateral meristem increases the width of plant organ so it is responsible for secondary growth. Lateral meristem includes vascular or cork cambium. Lateral meristems are both primary and secondary in origin (mostly secondary in origin). There are two examples of primary lateral meristem :
(i) Marginal meristem : It occurs at the margin of leaf. Its activity increases the width of leaf so total growth of leaf is called intercalary marginal growth.
(ii) Intra-fascicular cambium or fascicular cambium: This cambium occurs inside the vascular bundle. Except intrafascicular cambium all cambia are secondary in origin.