Buddhist:Why is it the Dhamma is said to be well expounded?

Answer 1

Because:
1.It can be seen and realized by oneself
2.It yields immediate results to those who practise it
3.It is worthy of inviting others to come and see it
4.It is worthy of being perpetually practised and borne in mind
5.It can be directly experienced by the wise,who have seen the Dhamma,each one for himself.

Answer 2

Because Buddha explained everything that needed to be said.

I say he explained enough, I won't go so far as saying he explained everything, but as long as you have a proper understanding of good, bad, and evil, that pretty much sums up the important stuff.