Why not egg?
Python’s egg format predates the packaging related standards we have today, the most important being PEP 376 “Database of Installed Python Distributions” which specifies the .dist-info directory (instead of .egg-info) and PEP 426 “Metadata for Python Software Packages 2.0” which specifies how to express dependencies (instead of requires.txt in .egg-info).
Wheel implements these things. It also provides a richer file naming convention that communicates the Python implementation and ABI as well as simply the language version used in a particular package.
Unlike .egg, wheel will be a fully-documented standard at the binary level that is truly easy to install even if you do not want to use the reference implementation.