A lot of it is that gg1 was a perfect playground for Cadmus. It encouraged close brawls a lot whereas gg2 doesn't. That combined with the summon changes means eyes and ears/berserker husks that have been summoned are a lot less important for your opponent to kill as they can't threaten strategy markers anymore, leaving the opponent more free to try and focus on the bigger guys.
Another reason is that of the really successful Cadmus players leading up the worlds, only Longtin actually stuck with explorer's where the rest of us went back to our main factions. As for it not happening in r1, Cadmus isn't the biggest fan of corner to start with, and then it being btl as well is rough, especially against Bayou where you have to worry about Zipp.