Why attack submarines resemble capital ships more than cruisers

Are nuclear attack submarines the modern warships that most closely perform the traditional cruiser role?

No; their wartime focus, expense, and inability to conduct visible presence or diplomacy make them closer to capital ships, though submarines remain a category apart.

An SSN can scout, attack ships and submarines, and sometimes strike land targets, but it cannot provide visible presence, command facilities, or broad air defence. Hiding from aircraft is normally its protection rather than engaging them as a surface escort would. Its expense and wartime focus make it more analogous to a capital ship than a cruiser, while frigates handle cheaper peacetime diplomacy and presence missions. Carriers and amphibious ships count as capital ships because they project power. Attack submarines approach the category because their value is concentrated in war fighting. Their operating environment and missions still make submarines a distinct group that resists a neat surface-ship analogy.