There is so much to this. There is the fact that an Air Canada has begun disallowing suitcases that people have used for years. (My wife has used hers on AC for 5-6 years and then last flight they said she couldn't use it after it slid into the sizer but was hard to get out.)

There's all the luggage chaos that was so widely publicized last year.

There's all the luggage fees.

There is the fact that every airline has slightly different standards.

There is the fact that a bag that fits in the physical sizer may not pass the electronic sizer.

Put it all together, and it is understandly frustrating.