Energy might be a reason, even though I suppose the navy would be the last thing to run out of gas in the US.
More like "station time" being the watchword in current naval building.
A nukular (love the word) supercruiser with a boomer like "gold/blue" crew concept and automated logistics could basically stay on station forever.
Like a surface boomer, only with the task of not causing Armageddon, but preventing it (or better, preventing 50% of it
)