why 2,2-dimethylpropane is not called isopentane? since 2-methylpropane is called isobutane
In common nomenclature, the prefix 'iso-' is applied when the chain of carbons is continuous bar one methyl group. The example you provided, 2-methylpropane, is observed to be continuous with exception to one methyl group bound to the central carbon atom.
However, 2,2-dimethylpropane does not feature an analogous structure: because two methyl groups are bound to the central carbon, the same classification is inapplicable. In place, the molecule could be labelled as 'neopentane', in which the prefix specifies a continuous chain excepting two alkyl groups.
The above image is 2-methylbutane, or isopentane.
The above-shown image is 2,2-dimethylpropane, or neopentane
If you desire further information concerning the nomenclature, I would advise the following link, from which these images were extracted: http://www.chem.ucla.edu/~harding/IGOC/C/common_name.html