The move is a bit of a surprise, given that less than a month ago, Inslee was reportedly telling Democratic operatives that he had no intention of stepping down. But as David Jarman wrote at the time, there was a reason for Inslee to stiff-arm the media and wait just a little bit:
If Inslee does decide to resign, but he can hold out until early March (but before May's filing deadline), we'd enter a window where, under Washington election law, the primary and general special elections would take place at the same time as the regularly scheduled primary and general.That right there is key. Local and national Democrats were undoubtedly eager to avoid another special election in the Pacific Northwest this year, after having to spend well into the seven figures to ensure that Suzanne Bonamici would prevail in January's OR-01 special. (Looking strictly at presidential election results, that district was very similar to WA-01—both were won by Barack Obama with around 61-62%.) So it makes sense why Inslee would want to wait until now to resign.
There will, though, still formally be a special to fill the last two months of his term, but as Jarman explained, it'll now happen in November. That means that Democratic efforts to defend this seat (which became redder in redistricting, dropping to 56% Obama) won't really change in a meaningful way. One possible quirk is that the election for Inslee's unexpired term would probably take place under the old district lines, while the election for the full term would use the new borders; it's conceivable, therefore, that the main candidates vying to replace Inslee for the long haul won't want to bother with the special, since they'd rather spend time focusing on the new turf. (And most of it is, in fact, new: Only 48% of the population of the new 1st is the same as that in the old 1st.)
The cast of characters in the race is unlikely to change, though. Republicans have coalesced around Snohomish County councilman John Koster, who very nearly unseated Rep. Rick Larsen in WA-02 last cycle. The Democratic field is wide open and includes 2006 & 2008 WA-08 nominee Darcy Burner, 2010 WA-08 nominee Suzan DelBene, ex-state Rep. Laura Ruderman, state Sen. Steve Hobbs, state Rep. Roger Goodman, and businessman Darshan Rauniyar. Oh, well, I shouldn't forget that there is one possible new entrant: Dennis Kucinich. Who knows? Maybe this new development will inspire him to run!