I really like the "Meitantei Conan" anime, but I've always wanted to know, why is was the name translated to "Case Closed" when it was localized?
Whether or not Detective Conan was catchy enough isn't the cause for the change in name in this particular case...
At one point Funimation's "Case Closed" website had a FAQ page with this question asked. An archival copy of this page can be found here.
Q: Why did you change the name to Case Closed?
A: Due to legal considerations.
We see the same vague response in a 2004 news article published by Anime News Network:
FUNimation sent out an advisory today stating that due to unspecified legal considerations, Detective Conan is to be renamed "Case Closed" . . . Due to legal considerations, this highly anticipated series will be released in the United States as Case Closed. Working closely with our Japanese partners, Case Closed was chosen as the name best suited for the U.S. market.
While it seems to be mostly speculation, there have been forum posts mentioning that this renaming is due to the copyright held by DC Comics on the name Conan. I'm not convinced by that reasoning, but I included it for you to digest as you please.
Conan is a name in the US that is closely associated with Conan the Barberian, and Conan is a name that sounds rather odd, the main reason why Japanese names are almost always changed in the US, e.g Kudo Shinichi to Jimmy Kudo. I believe the main reason other than that is probably for a more eye-catching name than simply Detective Conan.
What other people said is correct but I also believe they mention the phrase "Case Closed" in multiple episodes of the TV show/Anime so I guess whoever titled was like, that would make a pretty good title.