This is a great question! I do not raid and here is my 2 cents on why:

1. The raiding community in this game seems to only want people who already know the fight. The vast majority of party finder groups I see simply do not want people who do not already have experience in the raid. This creates a scenario where you have a limited number of people who have experience and a large group who do not and have limited ways to aquire it.

2. The raiding community in this game can often come across as stand off-ish or elitist (either rightly or wrongly). This causes players like myself to not be all that interested in being a part of that. I work, have 3 kids and so play this game for fun, not as a way of life. It stops being fun when it becomes about finger pointing and placing blame when things don't go well.