The key difference between these groups is the community that each one exists to attract and serve.
Writers is trying to create a community of writers, so it invites questions which will interest writing experts.
ELU is trying to create a community of linguists, etymologists, and other academic experts, so it invites questions which will interest these academic experts.
ELL is trying to create a community of language teachers and learners, so it invites questions which will interest teachers and learners.
To put it another way, consider how a grammar question might fit with each site:
If it boils down to writing advice (which construct will be easiest for the reader to comprehend) it is probably an ideal candidate for Writers.
If it gets more into linguistics or etymology (how should something be parsed, or how did the language evolve in this direction) it is probably ideal for ELU.
If it is about a common problem faced by English learners (what is the natural or idiomatic way to express this thought) it is probably ideal for ELL.