The reason is that so many people tells us that it doesn not work.
First, yes many people do freak at trying something else. They run away screaming, or send you to a psychiatrist - or a priest. I am not saying anything about how often this happens, but according to posts here, when it does, it hits peple where it hurts.
And sometimes they blab to their friends.
Secondly, if the 'conversion' does not spark a hitherto hidden need for bdsm, the best you can hope for is something done out of kindness or love, but with no real sense of the thing behind it. Ask a number of subs how that feels, we've heard about that too.