Say that I have a table of People, and these people have a Country column and a unique Primary Key. It's a demographics table, so these are the only things I care about; what Country and how many unique people are tied to that country.

I am thus only ever likely to SELECT WHERE or ORDER BY the Country column; a clustered index on the Primary Key doesn't do me any good, I'm not accessing this data by PK, I'm accessing it by this other column. Since I can only have one clustered index on a table, declaring my PK as Clustered would prevent me from using a Clustered Index on Country.