I'm playing solo because of 2 reasons:
1) I feel that I'm not much of a challange for other players - primarily because I hardly ever manage to finish one of the huge scenarios, so I'm unable to grasp the bigger, strategic aspect of the game. Plus I'm somewhat unwilling to delve into the whole reinforcements and strategic decisions-topic, so to a certain extent, I'm unwilling to grasp the strategic depth. I'm really focused on the operational level. I want to fight on the operational, not on the strategical level - I'd prefer if loosing a major engagement would make you loose the game. And I don't say all this because I'm greedy after winning - I just care about delivering a challenge and an open and exciting game for both contrahents.
2) I don't want to bind myself for too long. The current scenarios are all so big that I think it's quite impossible to finish them in a PBEM-game. I don't want to slip away silently and leave my contrahent behind in case I don't have any time anymore or in case I suffer a "motivation-low" (which happens...)
Smaller scenarios would give me much more confidence for playing multiplayer - I'd really love to play against other human beings.
I think that this might be true for all the people suffering from an "unstructured" life/work situation. But scenarios with more than ca. 60 turns and in which I have to command more than 2 armies, not even to speak of factions!, intimidate me too much. I have to get the feeling that the number of turns is limited and that the end of the scenario is not too far away. I don't know why it is. But this idea motivates me a lot. I own NCP, WiA, RoP, AJE (and megalo-bombastic PoN), but NCP and AJE don't hook me too much topic-wise. RoP would be my favourite setting but unfortunately it's the game with the biggest lack of small scenarios and it has the "march to the sound of the guns"-feature which makes me a bit sceptical because I don't fully understand it. I might try to find a relaxed WiA partner for some short scenario after I've finished my diploma thesis