People back in the day only sat D1 because they were, let's say, mathematically challenged compared to their peers. I don't mean to generalise here but it's kinda true. Remember that someone could easily sit M1 instead of M2/S2 to make up their A-level grade. As a result, even though D1 is a pile of toss (albeit, to be fair, a pile of toss where you have to explain your methods better and clearer than in any of the P/C/S/M modules), the grade boundaries were low.
I can only assume that centres cottoned on and exploited it. Certainly my college did back in 2005. Better quality candidates sitting the exam = Higher grade boundaries.