I think it is because he was seeking new ways in which to argue for the relevance of the Christianity of church doctrine, rather than going back to the scriptures from which it claims to come and from which it derives its meaning.
He was surely right to understand ideas about God as metaphors. He noted how 'up there' idea of God in the Bible was superseded by the 'out there' idea of God in nineteenth and earlier twentieth century theology. He, in turn, wanted to replace that with a new metaphor based on depth.
The first problem here is that this removes Christianity another step away from the Bible. The less the Christianity of the theologians connects to the language of the Bible, the more abstract and unreal it becomes. So the Christianity of 'depth' which Robinson advocates, impressive though it might be, comes across as an intellectual construct of dissatisfied theologians. It is interesting how the blunt message of God given in Jeremiah 22.15-16, which Robinson quotes with approval, contrasts with the abstractions all around it.
The second problem is that the 'up there' metaphor for God is by no means the only one found in the Bible. In fact there are many. It is thus misleading to present the Bible as if is is offering only one (obviously antiquated) paradigm.
If, as is surely correct, all talk about God has to be understood metaphorically, surely it is more satisfactory to work with those metaphors you have, rather than inventing new ones even further removed from the source from they ultimately originate and to which they must relate than the 'out there' idea of God that Robinson, rightly, wanted to dispose of.