I think the primary reason is so that the words of their magesterium and their "traditions" can be placed ABOVE the Scriptures in authority. They aren't alone in this faulty thinking, though. That's why we hear on these threads from so many about how we must thank the Catholic Church for our Bible because "they" wrote it, preserved it, compiled it, copied it and translated it and, without them, we wouldn't even HAVE a Bible. All poppycock, of course, but it seems to be the standard answer whenever the subject of the sufficiency of the Scriptures comes up.
I suspect the reasoning behind such talk is that they read what some of the Scriptures we post say and they can understand clearly enough that either the Scripture is wrong or their church is. Some of us decided it was the church and not the Bible. Others haven't gotten there yet - if ever - so they cling to their church in the hopes that someone will have a logical explanation for the contrast. Sadly, there will ALWAYS be someone around eager to do that and it's surprising how little it takes to settle them down (read some of the ScriptureCatholic references for a taste).
I'm so glad God found me and opened my eyes to the truth of the Gospel. He can do it for anyone who diligently seeks him.
Right on the money. Good post.