I wonder if it's partly because everyone pitched a fit over the battery status API, choosing to delete it entirely instead of modifying it, because it gave a bit of tracking information, and nobody supports it aside from Chrome and a few others that have not removed it(One reason I like Kiwi).
That almost seemed to be some kind of turning point, it was one of the first features to go in Mozilla's new relatively anti-feature mindset.
Now the trend seems to be to not even risk a few bits of tracking information, and people might not like the extra bit you would get from having the feature enabled or not.
Also, this is basically what Google's AMP is all about, and people seem much more interesting in hating it than making a better version.
If the choice is between simplicity and preserving real world resources(User time, SSD wear, CPU, bandwidth, etc) devs unfortunately always choose simplicity.
Devs are not limited to 4GB a month of data, nor do they have low end phones.
Also, social platforms have a bit less competition. There's no alternative to reddit except maybe some weird client app. The community is tied together with the tech, it's not like a flashlight where you can choose any model and light up the same place.
People want the fancy high bandwidth stuff, and nobody cares enough about performance to make a whole separate low bandwidth version.