Because if #2 and #3 come to be, then #4 will surely severely censure the police for firing tear gas, bean bag bullets and rubber bullets, which will demoralize the police, greatly damage their ability to maintain law and order and incentivize rioting as the proper form of making demands going forward. Supposedly Hong Kong's laws/ordinances governing the conduct of independent inquiries contain provisions against self-incrimination, which is possibly another means for rioters and their organizers to safeguard themselves against prosecution by presenting at the independent inquiry. I say "supposedly" because while chapter and verse were quoted, I haven't looked them up myself and IANAL anyway.
The "protestors" are demanding non-negotiable acceptance of all 5 demands.