There reason for the placement is due to the meaning of ML 2 vs. ML 3. ML 2 is all about stabilizing projects and gaining control over project estimates. Once the organization has achieved this, then it can begin to evaluate how to improve the engineering areas.
Since you need to have a baseline upon which to plan a project and the other ML 2 Process Areas, that is why REQM is the first Process Area in ML 2. The intent is to manage the collection of project requirements: good, bad, or indifferent. And use this collection to plan the project, etc. Then when you have achieved ML 2 and move to ML 3, then you can address how to improve the Requirements Elicitation to obtain better requirements.
Please keep in mind that the CMMI is a collection of guidelines and best practices for doing process improvement. The CMMI is not a roadmap for how to do software engineering.