There very may well be other reasons, but a large one is that Eclipse is written using SWT, not AWT/Swing. SWT is Java, and has a platform-independent API, but

There very may well be other reasons, but a large one is that Eclipse is written using SWT, not AWT/Swing. SWT is Java, and has a platform-independent API, but has platform-specific implementations. (SWT JARs are unique per OS/architecture)