Just speculation... Plenty of Indian engineers living in SEA - how do they get home? Many large multinational companies based there including Microsoft with worldwide travel needs. Most legacy carriers serve SEA including BA, LH and I believe SAS - so, more international traffic to poach. Plus, maybe just use a 777 or A350 (eventually) on the SEA route initially if demand does not require an A380. SEA could be considered a "long and thin" route until demand builds.
Thanks for the update Showel.