Because 1.42 uses a module named html and 1.41 does not. In any case, as is stated here:
http://calibre-ebook.com/download_linux
If you want to run calibre self-compiled from source, you are on your own, I really dont have the time to help with the myriad problems you are likely to encounter. If you want to run from source, use the officially supported way using CALIBRE_DEVELOP_FROM as described here:
http://manual.calibre-ebook.com/develop.html