Because pip doesn't recursively install the requirements defined in a package's requirements.txt file when it installs the package.
git clone https://github.com/kyrus/betterpip.git
mkvirtualenv / workon <your virtualenv>
pip install .
python -m betterpip install <any pip install args>
For example:
python -m betterpip install nose
python -m betterpip install -v nose
python -m betterpip install -r requirements.txt
python -m betterpip install --build build-dir -r requirements.txt