WHY TEETH REMOVAL FOR BRACES??

In most Orthodontic cases teeth are removed preferably all first pre-molars with the help of braces.for want of space to take front teeth back with braces.Why so?

During the treatment with common braces, only from six or eight teeth are taken back. The front six teeth will have only single roots which are narrow and small; in comparison to the teeth after them. During braces treatment in general all the teeth will not be moved back. It is because, in the upper jaw behind the last grinding tooth (wisdom tooth) there is an end to bone. In the lower jaw, there is a upward curvature of bone. Both these bone structures 
do not allow the all teeth to move back. Another reason being all back teeth will have more cusps(projections on chewing surface); so adjusting in acceptable position is really difficult if they are moved.We can expect maximum 1-2 mm movement in a month. Moving all teeth far behind will take very long time. To make the treatment possible, to reduce the discomfort to patient, to reduce treatment time and to give good relation between upper and lower teeth; teeth are removed in the middle of the row and all front teeth are moved back.