He needs power and knowledge to destroy the World Government and the Celestial Dragons, and Wano has plenty of both, considering that the country has deep connections with the mysterious Void Century and the Poneglyphs, which were actually crafted there.
Since no one else has been seen using anything similar to his Santoryu style except for Jigoro while he possessed Zoro's shadow , it can be assumed that Zoro invented not only Santoryu but most, if not all, of his sword techniques.
Ushimaru entrusted his friend, Monkey D.