Because of this most people don't contribute to android ROM projects anymore. For a lot of people it just isn't worth the effort those that do care tend to keep up with new tech and buy the latest and greatest tech so they aren't maintaining old devices as much anymore unless they were major hits or a personal preference of the maintainer.
As for why a lot of features have disappeared, through the whole process of surviving the Cyanogen Inc. issues, it took everything the community had to keep basic android supported let alone all the extra features considering how many devices they support. Now that Treble has been deployed, this shouldn't be so much of an issue but maintaining so many old devices that have long been deprecated isn't easy.
Also, as for why the S7 is no longer maintained, apparently there was some bs issue with the software license on the S7 binaries.