Neil Young was a good headliner when he did it about 2009 or 2010 but even then I don't think he had the biggest of crowds and id say the demographics of the festival have shifted since then.
Also, being realistic, as much as I like his music, the majority of people can recognise about a couple of his tunes at best. It's not the fact he's old, he hasn't got that many big hit tunes, unlike a McCartney or Elton John, who most average music listeners can recognise plenty of their tunes. Neil Young is a legend in music but he's quite niche in a way, in comparison to most headliners who can knock out a greatest hits set that almost anyone can recognise.
Some would say it's a good thing having a headliner like that, but id say most of today's festival goers would prefer someone for a good sing along.
But then considering the lack of guitar bands that are considered pretty new, there's almost none that are big enough to headline really.
So if you want modern acts that are big enough to headline the festival, you've really only got pop artists. If it's not them, then they mostly have to turn to old farts like McCartney and Young, or the usual headliners like Coldplay, Arctic Monkeys etc
As for the likes of The Vaccines and Bloc Party, although they're decent festival bands, some of them can barely sell out academy sized venues on their own tour. It's not easy to tell people to stump up Glastonbury ticket prices and some of your main features are bands that can't sell out a major arena and you could see at festivals like Kendall Calling for less. So I get why they don't really favor these kind of acts over others.
