Yep! It's surprising to me, too, how recently (within the past 4 years, by my estimation) there has been an intense, renewed interest in sixel.
It has coincided with the creation of a lot of new terminal emulators (some of the youngest have the most support for sixel and similar features), and with the trend of using GPU acceleration in terminal emulators.
I couldn't exactly say 'why now', but one guess is that macOS' explosion in popularity among developers and the relatively high popularity of desktop Linux within the developer community in particular have produced a generation of developers who appreciate the power and flexibility of the Unix command line but have also grown up with rich graphics on their computers from the start.
There's clearly a strong contemporary desire for a more IDE-like terminal experience, and for graphically enriched tools that are still decidedly text-centric. Efforts to establish new standards and protocols for this have mostly withered or seen little uptake, so resurrecting support for (today) rarely-used features of old physical terminals has emerged as a viable approach for adding graphics to our CLI environments.
Maybe some day we'll escape from the 80s, but for now it seems that we are still returning to them to find 'new' material, for various practical reasons.