Question: Instead of Frank giving the wallet with the fingerprints to John in the future, why didn't Frank just give the wallet to the police in his own time?
Chosen answer: They couldn't be sure that the police wouldn't lose it or turn it in when the owner showed up. Or, since we find out that the killer is a cop, he could have just taken it back from evidence.
Answer: Too much explaining would have to be done by Frank. The police would ask why he would want his wallet scanned for prints. Plus remember they were on a time frame. Julia was going to be killed in a week, and they were trying to save the other girls as well. Quicker just to get the wallet to john, and he would go scan for fingerprints right away.
Answer: Plus, they didn't have some of the technology in Frank's time as they had in John's.
Answer: Because in 1969 they didn't have fingerprint analysis.