Unfortunately, consumers in general were more excited by flashy new features that were of marginal utility as opposed to faster, more efficient operating systems so companies like Microsoft, Apple, Google, and many others devoted the developer time and effort towards an ever expanding feature set than towards improvements under the hood. Those still do happen, but for whatever reason customers don't view those changes as a compelling enough upgrade.
Now that the free ride Moore's law was providing for so long has effectively ended (regardless of whether transistor counts can still increase exponentially, the cost of doing so to the end consumer is also growing exponentially now as well when traditionally it was only linear leading to some amount of computational power or memory capacity become cheaper with time) the developers have to start pruning back the bloat. It's hard to add more bells and whistles when the previous bloat has already filled up memory.