I mean new models come out every year. With brands that only carry a few different models, they tend to keep a couple of generations (apple being an example). Then you have brands like Samsung or Xiaomi, that have a ton of models, they are not going to keep stock of the previous generation and why would they, they have models for every price bracket and so have nothing to gain, where as companies with a few models can keep older generations in stock at a lower price.