Because Sun made the boneheaded decision (and never admitted it was dumb) to go for emulated widget controls instead of native ones. What this means is that they have had to reimplement and imitate the behavior of native controls on all platforms (realistically, Windows only) every time Microsoft came up with a new version.

In contrast, IBM took the native approach with SWT / JFace, and the interfaces written with that library look and feel infinitely more native and performant than Swing and JavaFX ever did and ever will.