Cal Poly didn't take the report of their student missing seriously. This gave the killer ample time to discard evidence, move a body, etc.
I think the whole jurisdiction over the case early on didn't help. Cal Poly PD shouldn't have been handling a murder case (but they didn't know it was a murder at the time). The POI's family lives in Arroyo Grande which has their own PD. The county, and the best equipped department at handling a murder case is the SLO County Sheriff's Dept, but they weren't even asked to be involved in the case until about a month after KS went missing.
All of the evidence that we know of at this point is all circumstantial. I think the various departments kept thinking they'd get a confession, or find a body, or find a murder weapon, or find DNA evidence, which never happened.
That earring with the possible blood stain could have led to another search warrant, I wonder which department botched that piece of evidence?