Most games have turns. Sometimes (like most of the boardgames we played as kids, chess, and many wargames) players "take turns"; one player participates actively, others are passive or may have intervention opportunities. In some, such as Diplomacy or Young and Lawford's Charge, players record decisions and reveal them together, using the rules to resolve the results. Most campaign games I have played are order based, but add a referee using the approach pioneered by Kriegspiel.
So what's wrong with turns? In a turn-based game, progress is suspended until every player has participated appropriately. If a key participant is unable to participate due to life event, either the game is suspended or the form of the game has to change to recover. For example, in this Diplomacy game a player had to drop out, causing a disordered turn and a change of player. Most games do not recover at all.
Is there a way around this? We shall see, but first I want to examine some other issues in conventional games that form an obstacle to durable, flexible campaigning.