Everything that's going to be presented by the prosecution at trial has to be shared with the defense before trial. The prosecution also has to share anything that could be exculpatory to the defense as well, even if the prosecution doesn't plan to introduce it at trial. That's known as the Brady rule or a Brady disclosure.
At the end of the day it just takes a lot of time for both sides to line up expert witnesses, get them up to date on the facts, receive and share the evidence reports, have experts review the scientific evidence, interview each witness, prepare your own witnesses to testify, etc. And everything has to be done in a neat and organized way so if there is an appeal either side can defend the work they did. Lawyering is 90% paperwork.