Well both AF and LH have expressed interest in serving SEA so one should expect to see service to both CDG and FRA in the near future. The problem with SEA is that, as stated by others, it is does not have the feed of SFO, LAX, YVR, etc. Furthermore, its position at the far northeast of the US means that it cannot work as an effective transatlantic hub except for north-south west coast connections. As a result, airlines flying into SEA must rely on a greater degree of O&D traffic than at places such as DEN. This I would argue, is what has prevented more European carriers from serving the airport.
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