I don't see the answer to the question in the Wikipedia page nor in the first Google hits. Ok, Dooku did have a Darth name, but why wasn't he known by it? The Star Wars wiki has an extended biography that may contain the answer (something about using his nobility status for a power base, I think), but it could use a summary targeted to this specific issue.
On the other hand, given that the question has a massively upvoted accepted answer which only mentions the existence of the Darth name and does not go into the why at all, this Q&A is not providing any new content on the Internet or making any content more accessible. This is the very point of the general reference close reason: if we're not improving on Wikipedia, the thread is a waste of time. I say kill it, and let the question be reasked (insisting on a proper answer) if someone cares.