Why fast battleships replaced battlecruisers

Why did fast battleships displace the battlecruiser concept?

Higher engine power density reduced the protection sacrificed for speed, allowing fast battleships to take on battlecruiser tasks.

Improving engine power density reduced the protection and subdivision that a capital ship had to sacrifice for speed. A fast battleship could increasingly perform the battlecruiser's operational tasks while retaining battleship protection. Designers could instead spend that machinery improvement on still more speed, but no surface ship would become fast enough to outrun aircraft.