There are two sites for computer science: computer science SE and cstheory SE. Now, last I checked, doing computer science theory was and IS computer science.
What am I doing wrong?
From the help centers:
Computer Science Stack Exchange is for students, researchers and practitioners of computer science.
Theoretical Computer Science Stack Exchange is a Q&A site for professional researchers in theoretical computer science and related fields. We welcome research-level questions in theoretical computer science (TCS).
You are not doing anything wrong, there is just a split, and sometimes questions are migrated between these two sites (from TCS here because it was not research grade or when the question is research grade and got not enough attention here - but only good questions, the bad ones get simply closed).