I don't know that they're "
so popular" on TB. Seems about as popular as they've always been on here. Maybe a bit more interest lately since Korg bought them and gave them more visibility, especially on social media, and widened distribution (
Sweetwater, back at GC after some time, other retailers). They've also released new, (relatively) lower-cost import options that seem to be doing well.
Spector produce fantastic basses and their NS has become iconic. Their unique small, fully curved, arched body design influenced other manufacturers: Warwick's Streamer, of course, was initially a copy of the NS; Ibanez' Soundgear, no doubt took some influence with its sleek body, completely rounded edges and P/J configuration; and Yamaha (forget the model) also released a bass with similar design language.
They're well-made, have great finishes, a petite, curved body, distinctive modern sound. And while it can be argued they only make one shape with small variations, they cover a good amount of ground in terms of materials, finishes/colors, pickups/electronics, naturally through their U.S. custom shop, but also in stock offerings throughout their line.
I'm always blown away by their incredibly stunning glossy finishes and color creations. But being less flashy and preferring matte finishes, these are my boring, low-key Spectors

, and they're all keepers: