beacause that doesn't do what OP wanted. Quotes from the respective man pages:

• sort: -u does output only the first of an equal run (which means all distinct rows)
• uniq: -u does only print unique lines (which means all rows that appear exactly once)

[me@host tmp]$ cat /tmp/t
A
B
A
C
[me@host tmp]$ sort -u /tmp/t
A
B
C
[me@host tmp]$ sort /tmp/t | uniq -u
B
C
[download]