There are many, many causes dependant on each other, some with a bigger role than others.
I'm not inclined to try to summarize what books are written to cover in this post.
Suffice it to say, when you look at a core of people who did the damaging things, you get into the underlying setups of the system that increased the risk, from where you get into the lapses in regulation Wall Street lobbied for and won, and you get into the political corruption from things like our political funding system, not to mention all the well-intended people who just made mistakes - and all the people who played a smaller part doing what they were incented to (Wall Street wanted massive crap mortgages - and got them, people who could make money doing them doing so, buyers who could benefit doing so, and so on).
I'd say a basic issue was allowing money to have too large a role in our politics, limiting the oversight role government should have, allowing a lot of bag things.
If Wall Street could make a trillion dollars selling nukes to Qadafi, they would. While they did wrong, you can't look to them to say "oh, let's not make that money, it's wrong." The idea that you could look to them to do that was part of the spinoff of Randian ideology that said 'the rational market wouldn't cause a crash'. Wrong.