The Council gave the right to Absorb the Burrocks to the Adders for a single reason. To get the job done. Any other Clan? The Burrocks might have won. Only a few Clans had the ability to do so. And then there was the question of the absorbing Clan becoming too powerful as the Adders did become so. There was no interest in making the Wolves or Bears more powerful and the Spirits were considered to weak to have a chance to do so.
This is all headcanon, though.
First, Absorptions aren't assigned or voted on after the Absorption motion passes, they're
bid on, with the lowest bidder winning. The Adders had nine of their ten Galaxies participating in this Trial, which doesn't strike me as a particularly low bid. There's absolutely no way that if this had been written properly, the Clans wouldn't have been falling over themselves to get the chance to Absorb another Clan and raising a stink about it being assigned in the first place, whether it's because they're conservative sticklers (thinking Clans like the Falcons, the Vipers, etc.), have an ax to grind against the Burrocks/Adders (to the Jaguar POV, the Adders interfered in their Absorption of the Mongooses, and the Coyotes certainly didn't like the Adders), or are just plain thirsty for isorla.
Second, the Spirits actually had a larger touman that the Adders at the time of the Absorption: twelve Galaxies to the Adders' ten. They stood just as much of a chance on paper against the Burrocks as the Adders did. Granted, the Spirits would be at a disadvantage technologically, but I'd argue that that might be evened out by the fact that they have a lot less territory to defend, and thus could focus more into their offense. They gave as good as they got in the Absorption War (they lost five Galaxies but took down almost seven).
Even the argument that the Clans were concerned about the Absorbing Clan gaining too much power doesn't hold much water in this instance. The Spirits certainly wouldn't have become more powerful than the other Clans in this case, mostly because there likely would a lot less (if any) of the "let's just switch sides" we saw with the Adders: the Spirits and Burrocks
hated each other, and odds are the Spirits would've, at best, broken even because that Trial would've been
nasty.