Filbert Street was a bit of a dump and had a smaller capacity. That said, King Power Stadium was clearly done on the cheap and is about the worst of the "new" grounds I've been to (I've not been to the USM Bramley Moore Dock stadium yet...). Judging by the money their current owners have invested into training facilities I think they would have built a much better stadium - one suitable for hosting the Charity Shield!
Filbert Street was a dump but I went in 86 when Oxford beat Everton the same night. That was one of the best aways ever.
I think King Power could be big enough. Moving it to Wembley in 74 elevated the Charity Shield to a "glorified" friendly, with far bigger crowds. Before that, it really was just a friendly. I assume Arsenal were invited in 1971, but it ended up the FA Cup runners up vs the Second Division champions.
Move it away from Wembley and it's back to just being a friendly again.
For a lot of people the Charity Shield is a day out in London and a chance for the kids to see Wembley. But now it's a day out in Leicester, in July. I reckon quite a few can't be arsed with it.