I think the two comment above put together will be the best guess.
It looks as though only one side of the cutting has roots? If these roots where all around there would be plenty of roots at this stage to transplant into soil. It makes me think that the cutting was not of optimum health before the transplant.
Ther soil mix does look a little heavy in organic matter. Although these guys like lots of water they do grow best in a very gritty mix as aus has stated.
Put this together and I think that was the demise of the tree.
I remember looking at your cuttings in water Petra and you mentioned that you changed the water because it became stagnant. I never change the water. It is meant to have rooting ability so I guess it is concentrated by leaving the same water. It does sometimes need refilling though as the tree starts to transpire with the new leaves.
It is possible that the structure you had them in was not ventilated enough
Dunno but I would try again in a sheltered open position. Make sure the water is deep enough. At least double the thickness of the cutting might be a decent depth. Roots should be exploding from all of the trunk that is under water. As long as the cutting is healthy there should be no reason you could not plant the cutting at any stage of new root growth. As there is no reason you can't just do the cuttings in soil. I was just shown this way and I like it!
It will only be if you leave the cutting in water too long that it will start to suffer lacking nutrients.
Edit: it may even be possible that you left this cutting in water too long. Those roots look fairly mature even though there are not that many of them. It still should have survived though so I think it was also the after care that was the final blow ie, not the best soil.
It's too bad your in such a hurry cause the stories I could tell you, Bushels and baskets of stories, hole crates full of stories. But if you can spare a moment I will tell you one story.
As i had suspected 1 month ago, but thought it my strike back. 
It didnt & by checking the roots,noticed the've gone black and flat,there is no green left in the trunk at all. Every thing was still moist. Only solution is ive planted too early. Please let me know you comments. thought it had enough roots to support it self.
im interested. Guys.thanks for the replys.
