Watched a movie today about the Zodiac killer. Why is it that they cannot use modern forensics to identify the?

killer. Other murderers have been nailed with modern science even up to 30 years after the killing. I believe they do have suspects especially one that died at the age of 58. They have biological evidence. Anyone know anything about the Zodiac and why it has not been solved ?|||Back then they didn't have the technology they have today and not only that the Zodiac was clever he use to mask his fingerprints with airplane cement or something like that and didn't leave much evidence and Arthur the guy they suspected turns out they did a recent DNA test and he wasn't a match and his writing didn't match either. I don't think modern forensics can solve this case as there isn't much evidence to examine and after years most evidence becomes unusable.|||dna looses it viablity after a few years