In general,
Ethel is not seen very charitably. It's not just another "old fashioned" name like
Eleanor or
Evelyn, most people think it is actively horrible. It was a trendy fad name of older generations and often those don't stand the test of time. That's probably why it's not coming back.
In my area,
Candace is still considered boring-dated, like
Jennifer. It's a mom name now. I'm in my 40s and grew up with literally dozens of Candaces (spelled in almost any way you can dream of). Once this crop of Candaces become grandmas, it could come back. Give it 20 years.
Cynthia could come back a bit, especially with the rising interest in names like
Luna,
Athena,
Thea, and
Theodora. But maybe "
Cindy" is still seen as outdated? Maybe people figure if you're gonna go for a moon-goddess name, might as well pick
Luna (also, interesting note, I can't believe how few Lunas are actually out there, it's the #10 name but was only used 7,811 times last year?? Wild).
The sound of
Kirk is pretty different from the currently in-vogue K-names for boys, that could be working against it.