Because Bitcoin is not centralized, no one can decide that not even the developers. Bitcoin is slow, sometimes too much slow.
It also has some high fees sometimes, but that's not an excuse for any change like making it a settlement layer, simply put
You can never decide what to do with Bitcoin's code, that's the whole point of being decentralized, however you can use
Lightning network as the fast protocol and use Bitcoin as a settlement layer.
I mean, how to motivate more teams to develop various fast and useful protocols (Iot, scoial apps, etc) which will use Bitcoin as settlement layer? Usually teams are motivated by issuing their own coin/token and sell it to investors (shitty ICO model).