"If Numbers can sort rows why not columns?"
For the same reason that it does not sort individual coumns. Such a sort would not fit the model which Numbers is designed to follow.
Data in a numbers table is treated as a set of records. Each record consists of all the data in a single row of the table. The table can be sorted, but the sort cannot 'break' any of the records. Rows move as a unit.
Each column in a row represents a single field in that rows record. Fields may be rearranged (ie. columns may be moved), but the model assumes this is something that will be done in the course of setting up the table, will not be a continuing procedure, and will not usually require the fields to be in ascending or descending order using any simple sorting rule.
Design your table(s) with that model in mind, and thigs will go well. Work against the model, and you'll be frequently frustrated.
Should you require a different model, you have a couple of possible courses of action: Use an application that offers support for the model you require, OR convince Apple that the model they have chosen is too restrictive, and that they should change Numbers to fit your needs.
If the second option is your choice, then Provide Numbers Feedback to Apple, using the tem of that name in the Numbers menu in Numbers.
Regards,
Barry