Dick was also used as slang for Private Investigator/Detective. Mostly Pre-1970's.
The History of Dick
Why Does the Word "Dick" Have So Many Meanings?
Like I mentioned before, if Dick is growing out of favor, it probably has to do with the negative associations with it and/or just a change in preferences. The name Richard ranked #8 in the 1960's and now ranks #134 this decade [Government website data]. With fewer children being named Richard, there are probably even fewer choosing to go by the diminutive 'Dick'