Universal Studios needs him
For years now Universal Studios has been trying to reboot their classic horror movie icons in hopes of setting up some kind of shared universe to compete with Marvel Studios, but each of their attempts has ended in utter failure.
The Wolfman 2010- failure
Dracula Untold- failure
I, Frankenstein- failure
The Mummy 2017- possible failure
Guillermo del Toro has shown he knows how to make successful movies about monsters to make them scary and likable to the audience. Universal needs a visionary mind like his to map out how to reintroduce these creatures properly that pays homage to their original roots without alienating them by dropping them in a dull modern setting. He himself did try to make his own Frankenstein film that fell through, so its not as if he would turn down the idea of tackling this. Unfortunately business suits have no eye for talent and would rather just throw sht at the wall and see if it sticks.