I do recall that Degra had the Aquatics build the weapon because they were the most prudent (and probably the most sensible) of the Xindi species. (Also possibly the only race the others would accept weren't
going to use the superweapon against the other Xindi as soon as Earth was destroyed?). I guess he agreed that the project would never work if they were all bickering over who did what.
On that note, if the Xindi had destroyed humanity, I'm sure we'd all agree that they'd probably end up fighting each other over the superweapon.