Cost - Not only the main body of DruPal free, but almost all of its extensions are free.

Open Source - Friendly community support. Who need advice can easily plug into an intentional community of DruPal users who stand ready to help.

Flexibility - Everything, even its back-end interface is customizable.

E Commerce - Provides excellent out of box solutions.

Traffic - With special built in cache system, sites can handle heavy traffic.