If you ask me, I think it's fear.
As support, there are quite a few times when avodah zarah is referred to as fear/yir'ah in the Tanachic translations and the Talmud.
But on a deeper level, it's because of a weak connection with God. It works like a cycle; one has a weak connection to God(due to poor spiritual choices), which causes them to be scared because God took a certain degree of protection from him, and, in an effort to replenish their "holy" protection, they replace God with something else(chas v'shalom). However, it's all an illusion based on feelings. The difference between Judaism and other religions, is that Judaism is based on knowledge while other religions are based on feelings.
I could keep going but I think that's enough to get the point across. I think this is the true answer(in conjunction to the answer offered by the Talmud that the cause of avodah was to "permit" illicit relations) and based on a lot of thought. Just a simple answer I think could shed light on the topic.