Why have formal declarations of war have become increasingly uncommon in recent history?

Because they result in international pressure from organizations like the $UN,$ are subject to laws such as the Geneva Convention, and nations often prefer subversive means of warfare like cyberattacks.

Because leaders of nations worry about their public appearance and international status when formally declaring war

Because there is a fear that formally declaring war will lead to removal from international bodies such as the United Nations

Because warfare in the traditional sense is all but extinct, and has been replaced by subversive methods such as cyberwarfare and terrorism which do not require formal declarations of war