Possible reasons in my opinion :
1) As said above, might come sometimes from the fear of losing face on the IGS after a defeat. However, I do not think it is the majority of the cases, far from it. Even if some people may miss that point, as Silas said above, there is much more to gain from standing and fight instead of actually avoiding the fight. In a testosterone ruled world like Eve, trying to avoid a fight is being a pussy. Pussies are usually despised, even ICly. Losing on the other hand, can cause a lot various consequences, support or delusions from supporters, giving the enemy an edge IC or actually providing them a lot more unexpected enemies if you manage to turn yourself into martyrdom, etc.
2) As said above, which happens much more often, technical issues : either timezone problems, and/or just regional issues where the enemy is actually located at the other end of New Eden, which can cause a various amount of problems for some entities that are not very nomadic in nature or have a lot of assets to move depending on the scale.
3) What happens most of the time in people's head imo : the fear of the possible OOC/IC bleedovers and dramas that happen in a lot of wars. A war is not just shooting happily at each other. Eve implies material losses. A war in Eve implies consequences on morale, internal cohesion and synergy (<- not everybody can summon business cat to prevent that issue from arising), and more usually general OOC drama between RPers that get to know each other through other medias like The Summit or other RP related channels.