I know I'm late to the party but thought I'd add my 2c here. I think part of the difficulty is that in what you refer to as a toxic relationship I would call it a dysfunctional one. I think that stating it as a toxic relationship typically implies that you're with someone toxic for some reason thus placing the entire toxicity on them and none of it on ourselves. The fact is dysfunctional relationships are not void of our own input into the toxicity or at the very least the enabling of such bad behavior in others.

No matter how you will rationalize that they (the other person) was/is wrong and behaving badly, they have an usually continue to - give us something whether intentionally or not. if there was absolutely nothing in it that we gain by sticking around there would be very little difficulty leaving. Mind you what we get from it, from them, may not even be healthy or good for us (which is the most likely case) but somehow they feed or give something to us that we either subconsciously or consciously need. There is always a magnetism or draw of some kind and I think this is fundamental to why it is difficult to leave.

I hope that this simplistic explanation is not going to be taken as if it's the entire answer or that I think that's all it is. nor do I think it invalidates what others have said but I do think it's worth considering along with other answers.