Why would Christopher Steele, a respected investigator and former MI6 agent, a guy with a family and a mortgage and bills to pay, work for weeks without salary? Probably because he was convinced that the project he’d been working on (and which not enough people had paid sufficient attention to) was bigger than himself, bigger than just a job.
This Independent article by Kim Sengupta adds that “it is believed that a colleague of Mr. Steele in Washington, Glenn Simpson, a former Wall Street Journal reporter who runs the firm Fusion GPS, felt the same way and, at the end also continued with the Trump case without being paid.”