Why No English release

Because it's not worth for them. There is 42 Volumes, and the series was not that much popular in the West to carry the money for it, to the point they dropped the publication after Volume 16. There was rumors of a Remake of the Anime around 2020. If that was true, that could have pushed them to try to relaunch it, but seems it was fake.



Isn't there an exclusivity contract between Shueisha and Viz for Shounen Jump titles ? That would make it literally impossible for other licensors to get the rights... and if they could and were interested in it, they would have done so long ago (even if Viz is infamous for not wanting to drop their rights).

It depends.

Sometimes, they can still want to retain the rights despite not publicly exploiting them. Dunno if that's the case here, but as i said, i believe there is a contract between Shueisha and Viz anyway. So even if Viz doesn't maintain their rights for the series, it still doesn't allow other licensors to buy them.