I (think I) know quite a bit about the crusades after having listened to the history of the crusades podcast by Sharyn Eastaugh. And commenting on the fifth crusade today I realised that I have a theory on why the crusades shifted their focus to Egypt but I can’t remember if I got that from the podcast or if I came up with it myself. And if it’s the latter than it’s probably not the kind of stuff I should be spreading on /r/history. Anyways my theory is that they started targeting Egypt because it had a sizeable christian population which they could ask for support. In adition to that it had to do with the precarious position that the kingdom of Jerusalem/Acre was in. They were surrounded by the hostile powers of Syria and Egypt which were both in Ayyubid hands at the time (although crusader interest in Egypt started earlier with king Amauric). By eliminating Egypt they would basically eliminate their western front (probably anachronistic terminology), the areas beyond Egypt to the west and south were relatively poor or densely populated so taking Egypt would put the in a strong secure position from which they could push to the north-east. If they had gone for Jerusalem directly they might have taken it with more ease but it would have left them in a much less defensible position where the Ayyubid forces from Damascus or Caïro could attack the crusaders in the back if they were campaigning against the other.