Cultural inertia: same reason society moves slowly today even when a small group of people are convinced they have a much better way. When change requires the whole group, the change has to be clearly advantageous (high reward, low risk, or both). That's because there is a material and information overhead to change, with bigger changes being more costly.

It's easy and low risk to modify an arrow head a bit, so you see a lot of incremental progress in knapping. But moving the band from cave dwelling to tent dwelling would be a paradigm shift.