Politics.SE looks at the processes and policies of governments. Law is wisely keeping out of "specific legal advice," but can you fashion any questions that would be on-topic in law.se that is off-topic on politics.se?
Let's look at their descriptions shall we?
Proposed Q&A site for experts in the law, and those who want to find our more about specific local, national or international laws, or those who want to find out about concepts regarding laws in general. The site is not for specific legal advice.
Politics Stack Exchange is a question and answer site for people interested in governments, policies, and political processes.
Surprisingly, one site is about law and the other is about politics!
Law is, loosely speaking, about generally agreed rules that a country or community abides by in order to keep it in order.
Politics is, loosely speaking, the allocation of resources to a country or community. One of these resources being laws and their enforcement.
Law is a broad subject and it would be difficult for it to coincide nicely with non-law topics as found in Politics.SE. Law can be awfully specific.