If the Tianhe here is indeed the Convergence Point of Time Ultimate, then it's understandable for the War Hundred Clans from other timelines to actively help the War Hundred Clans of this timeline break this Convergence Point.
As for why they don't try to occupy here, the reason is not hard to guess. In other timelines, they can still develop well, so why must they occupy a timeline where the Convergence Point has been destroyed?
Just because it's the Time Ultimate?
That's the current situation, but once the Convergence Point is broken, it might not be the case.
Besides, as long as the War Hundred Clans from other timelines aren't foolish, they'll understand one thing: even if they occupy this timeline and eliminate Tianhe, what if they can't deal with the River Pioneers?
Who can guarantee the River Pioneers won't suddenly appear? Right now it seems he's trapped in Thunderstorm Land, but what about the future? There are too many unknown variables.
