Minimum Wage- Why just $9, why not $20 per hour?

In last nights State of the Union Address, President Obama called for a new minimum wage of $9 per hour.

If we are going to raise the arbitrarily obtained number, why not just go ahead and make it $20 per hour?  That way, no one working a minimum wage job would live in poverty.

Come on Mr. President, dream a little.  We know you think the government is the do all, save all for the American public.  So really, just $9?  $20 per hour would do so much more in your dream world.