Three reasons:

1) -d is not very descriptive, it's help "debug" is also too vague;
2) currently it only activates trace of FUSE exchange
3) in the next patch we are going to introduce -debug that will activate
   debugging-only features which will remain disabled on production runs
   for safety.

-> Rename -d to -trace.fuse to clarify its meaning and prepare for
   upcoming -debug.