That's not what the article said.
Here's what it said:
Prosecutors in the Robert Kraft prostitution case admitted in court Friday that there was
not enough evidence to prove there was human trafficking in the Orchids of Asia Day Spa investigation.
"Not enough evidence to prove" does NOT equal "no evidence".
The burden of proof in criminal court is high.
Often human trafficking is a "we know it when we see it" sort of thing, with little actual evidence to prove it, unless one of the girls is wiling to talk. Often the girls are afraid to say anything because they fear for their family's life back home, and sometimes their own life in the US.