I think you’ve answered your own question. 🙂 There’s no log2(x) because you can do log(x, 2). As The Zen of Python (PEP 20) says, “There should be one– and preferably only one –obvious way to do it.”
That said, log2 was considered in Issue3366 (scroll down to the last 3 messages) which added several other C99 math functions to the math module for Python 2.7 and 3.2.
Edit: log2 was reconsidered in Issue11888 and added in Python 3.3.