This question depends on what you mean by “why?”
Is this a question about the historical circumstances of how there came to be two creation stories and then why they were combined (i.e., the motivation in the mind of the editor)? That’s a question that I really can’t answer. One can speculate that they came from different circles in Israel and had legitimacy at a certain time for different groups of Israelites. Then they were combined later in order to provide a unified origin account for all Israel. That’s plausible but purely speculative.
However, the question I did try to answer is the canonical question of how to read them together (which could be phrased as the question of why they were put together–namely, to communicate a fuller canonical picture of God’s intent for the flourishing of the cosmos and humanity).
I would add that I don’t think it logically follows that one can’t answer the canonical question I did without first answering the historical/redactional question.