I can't say for sure, but it's probably something to do with reforms, arguably a way back to the old O-level difficulties (heck, some say more). And besides, it depends on what you call modern history, as A-G only started in 1976 (and the A* grade was introduced in '93).
I don't support switching it back. Now that I had to sit it, the next couple of generations have to suffer too.
Besides, with the exception of the last 5 or so in each paper, I think the new maths exams were well written. I didn't get an 8 or a 9, but I like maths and I like somewhat challenging maths... most of the time.
Although i think they are too hard for gcse level and ultimately unfair on students, i recently sat the new 9-1 english and maths gcse and my grades were considerably higher than i was warned they would be, i understand the grade boundaries are lowered but a 9-1 system is in place to distinguish and get a more precise result (9 is above an a* and 8 is an a*)
I feel lucky as I was in the last year that did all old spec GCSE's. Had a glance at the new spec's look much harder and horrible than the older ones. I sat my GCSE's in summer 2016 and so relieved that I didn't have to suffer the torture of any of those new spec's at all.
The new spec is not much harder than the previous spec. Anyone who was capable of achieving highly in the old spec is also easily capable of the same in the new spec. What the new spec is good at is differentiating the high-achieving from the absolutely stellar. For example, I achieved an A* in Maths (got 184/200). There's a difference between that and achieving 195/200 or more. This new system is good at that.
shouldn't change it now. Why should we be the years to suffer from the lack of resources and etc while the next years can get away? Keep it for a bit more years then change it.