agiastik@email.it pisze:

the answer is portability ;)

A more serious answer.  It is traditional that major development tools
for a specific language are written in that language.  So tools like
javac, NetBeans, Eclipse, javadoc, and many more are implemented in
Java.  Doing so has several advantages.  For example, it ensures that
people who are building Java development tools are aware of the
usability issues of those development tools... because they use them on
a daily basis.  It provides non-trivial testing, practically for free.  
It's also a rather expected show of confidence by the developers; if
major Java development tools were all written in C++, one might
justifiably ask whether Java is sufficient for such tasks.

My thought is that portability, an earlier answer, is secondary at best.  
It's honestly not that tough to write portable source code in most
languages, and at least Eclipse is built separately on each supported
platform anyway, because there's a native layer to it.

That's often called "eating one's own dog food":

   <http://en.wikipedia.org/wiki/Eat_your_own_dog_food>

A compiler or IDE that's capable of building itself is often called the
"dog food" release.

sherm--