Because they are internal APIs: they are subject to change in a undocumented or unsupported way and they are bound to a specific JRE/JDK (Sun in your case), limiting portability of your programs.
Try to avoid uses of such APIs, always prefer a public documented and specified class.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…