Because of the absence of cambium in vascular bundle in
between xylem and phloem no secondary growth is seen in
monocots.

But in monocots like Draceana Secondary growth is seen .
Only in some exceptional cases but it is rare.

exceptional case of formation of cambium in monocots is the 
reason for growth termed secondary growth.the general 
herbivorous habit might recollect the stste of intense root 
system  for firm attachment to soil.