There's the corporate side of it, which other comments have covered, but consumer mentality is a big piece, too. Seems like we're so awash in content there's a widespread jaded expert mentality that's taken hold. A lack of naive willingness to try new things, possibly paired with or caused by a feeling of being overtaxed financially from all sides and having too many things demanding our time.
A lack of willingness to spend time or money on something we don't already identify with as being good, on both the sides of consumers and producers.
Late stage capitalism has changed us all. Feels like there's a lot less room for experimentality in this huge carefully curated experience. We've all seen too much.
edit to add: Maybe the popularity of reboots are us yearning for simpler times. We can't reboot society so we reboot our movies, music, shows, etc. Meanwhile, constantly rehashing old plots prevents the renewal we really want.