Lack of money from developers/ owners of the land, better opportunities developers are working through first, the plans approved are too expensive/ or not workable to build, speculation growth (where actually its more valuable to keep the land as it because it’s increasing in value), CIL, requirement for social housing makes the development non profitable.
Lapsed planning permission is another one, usually the planning is valid for 3 years, so the majority of of those approvals will have expired (assuming those approvals were equally split across the past 10 years). A site with previous planning means it’s more likely to be approved again, but still have to go through the process.