How can a pilot involved in serious accident remain unknown? Somewhat similar to this question. A short part from Wikipedia article about Emirates Flight 521 (emphasis mine):

The captain was an unnamed 34-year-old UAE national who had been with Emirates since March 2001 and had logged 7,457 flight hours, including 5,123 hours on the Boeing 777.

In which situations or under which circumstances a name of pilot involved in serious crash may remain unknown or hidden?
In this case, it's most likely that the source for the story (the airline?) simply chose not to name the pilot and the editor of the Wikipedia article didn't seek another source that might have done so.
It's virtually certain that the pilot's name appears in the official reports of the incident.  The airline likely chose not to give the name for PR or legal reasons.