Its not a 'law'. I am amused to see that its stated as laws in every god damn engineering text books. and my friends byheart it and write the figure and boost that they got full marks for the question
Well its an prediction by Moore who used to be a Intel Engineer/Co-founder. Practically ULSI/GLSI and above transistor integration has limits due to power dissipation, loading effect etc etc And in the future say 40-50 Years from now, i think we may have a replacement technology to good old transistor.
*en.wikipedia.org/wiki/Moore's_law