>>new to me
The wrangling over it took forever. For multiple years, there were competing proposals on how it would work. Because of that, and the relative complexity of implementation, both from a browser and developer standpoint, adoption has been fairly slow. The biggest obstacle at this point, though, is mostly the complexity of implementation from the maintainer/content creator/editor point of view.
Initial discussions date to 2007, took a hiatus, came back in 2011 after Ethan Marcotte coined the term "responsive design" in 2010. There was a W3C draft spec released in 2013 at the latest. If you were subscribed to A List Apart back in those days, there was a lot of discussion. There was a long, sordid history of whether we would get <picture> or just the srcset attribute. At a certain point, a lot of people started tuning things out as W3C and WHATWG and RICG were arguing over not just details, but fundamentals.
If you're curious about the nasty stuff that goes into web standards sausage making...
https://thehistoryoftheweb.com/responsive-design-picture-element/http://alistapart.com/article/responsive-images-how-they-almost-worked-and-what-we-need/On a more practical level...
Two sites I maintained, built around 2013 and 2016 if I recall, required uploading three custom sized images for every image used in certain contexts. In one case, every page had a header/hero image and the widescreen version had a super wide aspect ratio (over 3:1 if I recall - 1920 x 580 I think). The tablet version was roughly normal landscape aspect ratio. Small (phone) resolutions were square. That meant not just custom crops, but often completely different images since what crops well at 3:1 usually looks awful when square.
It can turn basic site maintenance and updates into a nightmare task if you let designers get carried away. They come up with a client-wowing design that looks amazing in the two mockups they do. But then the reality set in of having to find 2-3 completely different images every time you want to build a page, then crop them all separately, then upload them. I find that if you're looking for images with zing, it is a huge huge part of the work on a page and when you double or triple that work, it can bring the process to a crawl. If you are using stock images, it makes finding them super hard. If you are doing photoshoots, you need to explain your needs to the photographer and find a photographer who can think not must about the compositions in the photo as taken, but the potential to work at different crops and aspect rations.
So be careful about having obligatory <picture> elements as part of your overall site design. You can live to deeply regret it.
