To be honest, I think it was the availability of wealth and comfort. Perhaps we can use food as an analogy: a person can get too much of it, and get fat and lazy and have a heart attack. A healthy life requires us to balance food with a proper diet and exercise. For us, food is easy to get, whereas diet and exercise demand willpower -- which, if obesity statistics are any guide, Americans seem to lack.
This, imo, is similar to what happened in Rome: the Roman citizens were able to enjoy their wealth and comfort, and were insulated from the need to understand, much less defend, its source. They "outsourced" their defense to barbarians, for example -- no longer did they have to worry about defending themselves, beyond (maybe) paying some taxes (and I don't think Roman citizens even had to do that).
I think people that are enslaved DO yearn for freedom. It is those who are coddled that choose security.
I'm not sure enslavement is necessary for this -- I think that the necessary civic virtues are strengthened where there is a clear connection between our own efforts, and the results of those efforts; and weakened when that clear connection is lost.
I'm really thinking here about the bleeding idiots who say things like Iraqis aren't ready for/don't want freedom. Invariably said by people who do not have the empathic skills, because of wealth and plenty, to imagine what life must have been like under Saddam.