3 ideas
Read any fresh political beginning as sited on top of older violence and deals it would rather not excavate. Construction and renewal disturb the ground and surface what the old order hid. Legitimacy then depends on whether those remains are investigated or quietly reburied.
When the victim is a rising politician, everyone around him has reputational stakes: family, party, rivals, and property developers eyeing the new regime. Witnesses shade the truth to protect careers and deals rather than the killer. The investigation is slowed less by lack of evidence than by the many people with something else to hide.
Rebus is assigned to a liaison committee for the new Parliament's security, a bureaucratic role built for cooperation and appearances, and he is unsuited to it by temperament. His usefulness comes from refusing the committee's logic of smooth transition and pulling on the loose threads officials want tied off. The outsider inside the institution sees what its procedures are designed not to notice.