Because you can take someones matter blood etc, and do a DNA profile that is virtually foolproof. Just like they do when a child's parentage is in doubt. You can take someone's fingerprints and compare them with those taken at a crime scene to see if you have a match. Hard evidence.
Bug stuff is a matter of opinion that can be "skewed" by many variables.
I believe you are knocking a useful science you know nothing about based on your limited knowledge of one case you followed in the news. I challenge you to educate yourself more generally about forensic entomology.