Product (non) availability..whats up with that?

I just recently started looking at aritzia stuff. I find most of the things I like not available in the larger sizes L or XL, or not available at all. Is the non availabililty a product of the recent sales or is that a constant thing? Are they restricting availability to create demand or is there so much demand that they are short on stock. I'm so confused with all of this. Thanks for any input you can provide.

Super puff are but one example