why couldn't a Christian say evolution was Gods plan?
after all, why would you understand Gods plan?
It could have been (and in part was) - or God could have used all kinds of ways - including snapping "his finger" and everything material is in existence with apparent age. I mean after all, omnipotence (rightly understood) is part-n-parcel with the idea of God.
We certainly can't understand God's plans, ways, actions, thoughts, etc., comprehensively; but only insofar as God reveals such to us in a way we can understand.
The question under consideration what the God of the Bible is revealing in Genesis (taking for granted of course that there is a God, and such God reveals himself to man, and has done so in/thru the Bible - given presuppositions of Christians - so those of you who aren't please bear with me). Are those early chapters what I would call a flat historical account, written according to some modern norms for historical accounts and a sort of quasi-scientific textbook (as ironically both the Ken Hams and Richard Dawkins of the world read it - such "fundies")? Or are they Ancient Near Eastern apologetic, literary stories of origins written against the backdrop of Egyptian and Canaanite stories of origins from the basic same time period, though containing both historical and "scientific" implications? As is obvious, I think the latter.
As the latter, what is being addressed is that Yahweh, the God who has entered into a covenant with man (particularly the Hebrews) is the Creator God. It's an attack on various other world views - atheism, pantheism, polytheism - and especially the polytheism of the Egyptian and Canaanites. it presents origins theologically and covenantally. Does it have historical implications? Sure. Scientific ones? Sure. But those aren't the point and greater care needs to be given when drawing out such implications, imho.
I think the Ken Hams and Richard Dawkins of the world miss the point and in an idiotic-silly fashion apply foreign norms and rules to the text. I wouldn't apply the norms of Metaphysical poetry to a textbook on basic biology. I wouldn't demand that a haiku be understood in keeping with Western writing of jurisprudence.