Canada

Blame FedEx, and Amazon.

So ... Once upon a time, you'd get to bring one or two checked bags for free. Then FedEx came along and you could ship documents overnight (except it was really expensive). But they started to bring costs down and it became more reasonable to ship stuff that way, at which point the USPS, UPS and other carriers had to start offering similar options or risk becoming the second or third choice carrier. But they needed a way to quickly move letters and packages around the country without having to buy an entire fleet of airplanes.

And then a bright person said, "Hey, why do they have to be our airplanes?" And they went to United and American and all the other carriers and said, "Hey, we want to buy space in your baggage holds, and we're willing to pay!" And suddenly the space under the plane where they tossed people's bags stopped being a way to lose money (did to the extra fuel needed for the extra weight), it became a revenue stream, it was profitable, and the airlines rejoiced.

But now every suitcase a passenger brought up meant less space for those lovely, lovely packages. So the airlines started restricting suitcases - number, size, weight, whatever they could do to create more room for packages - and the start of Amazon only meant even more packages that wanted space. And other companies wanted to compete with Amazon, and packages increasingly needed timely delivery to ever more remote parts of the country.

Anyway, the upshot is that - in addition to squeezing in ever more passengers - nowadays, when you pay to bring along an extra suitcase or oversized/ heavy item, the airlines are actually charging you some of the revenue that they're losing because that space is no longer available to carry lovely, lovely packages.