You should read up on exactly what a clustered index is rather than a regular non-clustered index and the difference will make it obvious.

Essentially a clustered index defines the physical ordering of the data in the table. Obviously the data in the table can only be ordered once - all other (non-clustered) indices store a subset of the columns in a certain order as well as storing the contents of each row as far as the clustered index is concerned. This allows the non-clustered index to do a "bookmark lookup" if need be to access the table/clustered index to retrieve other column values.