Sacks weren't an official stat when Harvey got his 23. There were other players, like Deacon Jones, who had 22.0 sacks twice, 21.5 once and 19 once. Now he is in the HOF, but he also played for 14 years, vs. Harvey's 11, and had 8 Pro Bowl appearances vs. Harvey's 5.
I agree Harvey deserves a spot in the Hall, but they won't put someone in based on unofficial stats.
I will say, with the election of Howley, there are to me no other Cowboys players whose non-election to the HOF is in the "it's a crying shame" category. Several other Cowboys' players should be in, such as Leroy Jordan, D. Woodson and Cornell Green, but you'd have to make a case for them to be in, and there are other non-Cowboys players who are in the same category...