From what I can tell, it was a full co-production, with Hanna-Barbera involved in the creative process from the get-go. The Ultraman Wiki says "Hanna-Barbera provided crucial advice and guidance in planning and production. In 1986, the project entered the specific implementation phase, with Mitsuo Kusakabe from a Japanese studio serving as the director, and the script being jointly authored by four American writers under the pen name John Eric Seward." (The writers were John Loy, X-Men's Eric Lewald, Jeff Segal, and Kelly Ward.)
So I don't really know why H-B or WB would only have the rights to the English soundtrack, when H-B had a hand in the creation of the entire story. I'm not convinced that's the reason for the new dub.
However, I checked the video that's available on YouTube, and it turns out that, unlike most American animated shows subcontracted to overseas animators, U:TAB's animation is not lip-synced to the English dialogue. It must have been animated the usual Japanese way, with random lip flaps done first and dialogue dubbed onto them later, or else animated to a Japanese dialogue track. Which would mean I may have been wrong and the English dialogue was a dub after all. Although that's strange, because the jointly owned rights meant that it didn't get released in Japan until a year and a half after its US release. It's all pretty weird.
So I don't really know why H-B or WB would only have the rights to the English soundtrack, when H-B had a hand in the creation of the entire story. I'm not convinced that's the reason for the new dub.
However, I checked the video that's available on YouTube, and it turns out that, unlike most American animated shows subcontracted to overseas animators, U:TAB's animation is not lip-synced to the English dialogue. It must have been animated the usual Japanese way, with random lip flaps done first and dialogue dubbed onto them later, or else animated to a Japanese dialogue track. Which would mean I may have been wrong and the English dialogue was a dub after all. Although that's strange, because the jointly owned rights meant that it didn't get released in Japan until a year and a half after its US release. It's all pretty weird.