Due to the fires and the tower collapses while we know people were in the building when it came down there wasn't a body that was found. In some cases the body parts that were found were extremely degradated and the DNA testing at the time and up until now isnt advanced enough to ID them.

Some parts also just dont have DNA and so other methods are literally being invented/created to link these remains with loved ones. It just takes a lot of time and waiting for scientific advancement.