Water heaters produced after April 16, 2015 must conform to new energy efficiency standards. These translate to a mandatory increase of 3% – 30% greater efficiency vs. current models.
2015 Government Changes to Water Heaters WILL Affect You - Washington Energy
Federal Regulations Will Dramatically Increase Water Heater Replacement Prices For Austin Residents
In a nut shell.
Many plumbers are buying up as many of the water heaters they can to stockpile.
New water heaters may not fit into existing access holes or spaces like old ones causing costly renovations to that space.
They will not only cost more, the installation costs may be higher.
If you think you will need a water heater in the next year or so it might be wise to purchase one now and store it until you have to put it in.
(for all you nit pickers, notice how I did not say 'Hot Water Heater', LOL)