A different time. The Klingons are on the move and Starfleet has to keep pace while still continuing their mission to explore. The Constitutions are the extension of their exploration policy while being able to combat Klingons in the space near the borders.
Prior to this Starfleet ships would have difficulty dealing with the combat oriented Klingon warships, as seen in Enterprise. It took two upgraded NX-class ships to even threated one D5 cruiser. An NX-class cruiser was barely able to match a old Bird of Prey or Raptor. The Kelvin era ships could likely handle the D4 and maybe the D5, but not the D6. The Ares was the short term solution to the D6 problem by building a combat ship rather than Starfleet's usual multipurpose starships. The Constitutions are the long term solution to the D6 and it seems the D7 as well, by using the tech from the Ares Project for the next generation of multipurpose cruisers and then increasing its power to make it a "heavy" cruiser.
*What defines a heavy cruiser from any other cruiser in Starfleet?
The term is a left over from the Washington Treaty to World War II era of cruiser construction. The difference between a heavy and light cruiser back then was the size of their main guns. Nothing else. A light cruiser could be as large or larger than a heavy cruiser, but was armed with 6.1 inch guns are smaller. A heavy cruiser was limited to 8 inch guns. All cruisers were limited (by treaty) to 10,000 tons. Most countries cheated a little on tonnage, but the gun limitations were held. You could have a ship that has 6 eight inch guns called a heavy cruiser and also a ship with 15 six inch guns called a light cruiser. The light cruiser will do the most damage in that case. mostly because the weight of all those shells, plus the 6 inch guns could fire rapidly (automatics) while the 8 inch guns has a slower rate of fire until the end of World War II.
