Why is the 2006 series the last date on the US 1 dollar bill?

It isn't. Bills have been printed with 2009 and 2013 series dates.

The reason some dates don't appear is that unlike coins, where the date is normally the year the coin was minted, US bills' dates are "series" dates rather than printing dates. Since 1974, a new series starts whenever a new Secretary of the Treasury is appointed or the bill undergoes a major redesign. The series dates correspond as follows:

• 2006 - Henry "Hank" Paulson
• 2009 - Timothy Geithner
• 2013 - Jacob "Jack" Lew