A lot of press hype about how bad they are has caused a massive drop in price, plus I think i read somewhere that the government want to make it more expensive to drive them over the next couple of years (road tax and fuel tax I guess?), new ones are not selling either, all the old folk who have always driven them have panicked and are all going petrol (I have a friend in BMW sales and it's hit his commission hard).
Ford have already dropped diesels from their 2021 lineup in favour of hybrid and petrol engines too so they won't be around that much longer if other manufacturers follow suit.