Consumerism, laziness and greed are some of the biggest causes, IMO.
Population is a huge issue, but some countries have a large population with a relatively low combined carbon footprint.
We've gotten used to an unsustainable lifestyle where we can have what we want at the click if a button, throw away things rather than repair them, "upgrade" items when a better one is available, eat as much of whatever we want just because we fancy it, jump in a car for convenience etc. There's a cost for all of those things in terms of sustainability, environment, and also the treatment of others who work for pence a day while struggling to make ends meet, so we can have the luxuries we've grown accustomed to.
But we're not only conditioned into that behaviour, but pushed into it as companies chase ever bigger profits. Items are made cheaply so that they're more difficult to repair, or not worth repairing, electrical items and white goods no longer have half the lifespan they used to, and technology has inbuilt obsolescence so that your perfectly functional 4/5 year old phone/laptop gradually gets slower and less useable with each update, until you're pushed into a replacement. And everything is focused on making us buy things. We're seduced and bombarded constantly, or simply told we're disgusting or dysfunctional if we don't buy a range of items. And individuals could overhaul their lifestyles (and of course, every little helps) but it's industries who have by far the biggest changes to make.
Not to mention too many people are struggling to simply making ends meet, to make other causes a priority.