"There aren't too many reasons for one private company to do due diligence on another private company besides acquiring it"
That is an often repeated but mistaken myth.
There aren't many reasons for a company to agree going through a due-diligence except for the chance to being bought, unfortunately some companies use due-diligence as a research tool into markets they think they might be interested in with no real intention of actually purchasing the inspected company, sometimes even developing a competing product at the SAME time.