Maybe one reason is because they've been around long enough to know what happens with bleeding edge technology.
And as the (old) saying went, "nobody ever got fired by going with IBM".
But the truth is the reliability and "shit hits the fan" if a Wall Street system goes down (and financial loss) is much greater in a traditional business system then if the same thing happens for a free service like facebook. Or somebodies "Show HN what I built this weekend" app.
So of course they are going to move slower. And they should. They have more to lose.