I'm just going to start by saying some of the best sliders in the world have come out of one skateshop and that is Motion Boardshop. Seattle longboarding has some of the strongest slide styles on the history of longboarding. The like of Ethan Cochard, Ross Druckery, Sam Galus, Jackson Wells, TJ Joo, Devon Dotson, and Max Wipperman just to name a few. So one has to ask, what was in the sauce?
https://youtu.be/s2B409UMnUU?si=jAnHGyIB8AapB_uy
Being a major metropolitan area with a lot of easily accessible hills are some key factors. Progression happens faster when a strong community is present, just having gnarly hills isn't enough if no one is there to teach and inspire others to skate them.
Also slide jam style freeriding was huge at the time, and Seattle has no shortage of straight and steep hills for that riding style.
@RobustOrange facts. A lot of groms pushing each other is a definite boon for progression. Especially with a whole bunch of different spots to skate that cater to different styles and stances.
Skating with other skaters makes u wanna skate more,
From my perspective, the whole freeride/slopestyle thing really kicked off in the Vancouver area. It seemed like almost overnight, hands-down slides went away and everyone was doing standies. Landyachtz and Rayne were both out of BC. Seattle is the nearest major US metro to that area, and has very similar geology - the same types of hills: short bangers, not necessarily the huge downhill tracks of your SoCal or your Appalachia.
2007 or 8 was when dudes started getting air - that was part of the impetus of Bear's 10mm axles on their Grizzly trucks, unfortunately the first 2 generations of Bear Grizzly had really crappy aluminum and the bushing seats would often break upon impact. This was also sort of the impetus for chopping boards, though that really kicked off a few years later. As dudes were riding like the picture below, the dropthrough cutouts or the drops in the deck would smash or break and they'd re-drill the trucks to the standing platform.
So, the answer (at least from where I watched it) to "how did the 2013 Seattle Scene beget so many good sliders?" is the same ol' story of skateboarding in general: It emerged from the general miasma of the time. There are multiple simultaneous claims to the first ollie, just as there are multiple simultaneous claims to the first kickflip on flat, and the first kickflip on vert. Skaters are a creative bunch. If this genre of sliding didn't evolve in the PNW, it would've evolved in New York or New Mexico.