Because it isn't meant to, at least not yet.
NopCommerce relies on Entity Framework which was recently open sourced by Microsoft. Mono has included it in their latest beta. It doesn't included support for MySQL database. MySQL driver for .Net has Entity Framework features turned off for Mono due to the same reasons.
What is available under Mono for Entity Framework provides more obstacles than assistance as not all features are ready and some features don't work as expected. Oh! And MonoDevelop beta that you'd use with Mono 3.0.3 beta, doesn't have functioning debugging hooks. So the code is sprawled with Console.WriteLines with nonsensical one - two word phrases to leave a bread crumb trail. But the code wasn't meant to be pretty either.
The admin portion isn't functioning yet due to an issue with generated SQL somewhere.
I'll setup a repository in GitHub soon to show the hacks that were put in place to make this possible.