For various reasons, most of which we've touched on, Glenfiddich is deemed by many people to be a beginners' single malt.
Which means that as soon as the beginners have got a few different drams under their belt, they branch out to other more exciting whiskies, and tend to cast a disparaging eye back to Glenfiddich. I think a typical philosophy goes something like this:
"Oh yes, that's Glenfiddich, the whisky I first started on. But I've matured since then, and I now enjoy other, more challenging malts".
I also think it's trendy for malt snobs to pooh-pooh Glenfiddich. (I can say that, because I'm a self-confessed malt snob). Why defend a common Glenfiddich when you can extol the virtues of your rare and exotic 1963 single cask of Bruichladdich?
It's like riding a bike. Once you've got your balance and you're confident, why would you go back to using training wheels? People adopt the same mentality with their scotch.
Make no mistake - the entry level Glenfiddich is the biggest selling single malt in the world, and the producers know exactly what their consumers want. It's no accident that it's not the most challenging malt on the planet, but at the same time, spend a bit of time seriously exploring this whisky, and you'll find that it offers a fair bit more than people give it credit for. I've had plenty of dull & unpleasant speysiders (Deanston 12yo comes to mind) that make Glenfiddich look like nectar.