I was a protestant for 30 years, I have a little insight. As my priest pointed out, during my instruction to the CC, at its basis, protestantism is in protest. This is the “you can’t tell me what to think” attitude. “We can vote for a president, why not on doctrine?”. We have a real problem with authority, maybe its left over from the 60’s, I don’t know. In America, we have dozens of churches at our disposal, maybe more. When we get the feeling our independence is threatened, we can make a change (of churches).
People always want their own way, see Genesis 3. We want to be like God, or be our own God. Therefore, rather than submit to the Church’s interpretation, many choose their own way. We have become so used to having things on our terms, that we have forgotten how to submit. I do not doubt the fact that the Holy Spirit works in non-Catholic churches and non-Catholic hearts, I have been there and seen and felt it. If the Holy Spirit was truly guiding everyone outside of the CC, there would be uniformity. Not necessarily uniform styles, but uniform doctrine. There are some who claim to be led by the Holy Spirit, that are working toward very liberal ideas, many of these ideas contradict scripture and Tradition.
I think many people, outside the CC, are well meaning and good hearted people. They are lacking the fullness of truth. They have been raised outside of the fullness of truth for generations, thus are unwilling to change. It is really sad, I hate to see this divide in Christendom.
To return to the OP, when you base your belief system on protest, it becomes an easy way to justify division. Once you reject the Authority give to the CC, everyone has an opinion and they all weigh the same. Somehow, by throwing away 1500 years, they think they can re-invent the first century church in 2012.
As we approach the feast of the conversion of St. Paul, let us pray for unity and conversions! St. Paul pray for us!