Because Bitcoin is focused so much on preserving backward compatibility, there are certain updates we won't ever see included in Bitcoin without causing a fork. Interesting approach by @decredproject to avoid this by unifying the community through a PoW-PoS combo.
https://twitter.com/theobtl/status/1019676848127664130This is no guarantee. Sure the changes are voted but that doesn't make them unanimous. In theory there could always be a chain split.
It is very unlikely of course