Complex sentence: Retro Hugos have only been given four times: in 1996,2001, and 2004 for 50 years prior, and 2014 for 75 years prior; the six Worldcons eligible in 1997--2000,2002, and 2015 chose not to award them.
Split into simpler sentences: Retro Hugos have only been given four times: in 1996,2001, and 2004 for 50 years prior, and 2014 for 75 years prior. The six Worldcons eligible in 1997--2000,2002, and 2015 chose not to award them.