From a bit of research, dont think this is a particullarly hot product, lots of identical items out there, all with a very low level of reviews, so suggests it does not sell in big volumes, same item is on eBay, Etsy, On the High Street etc. and each one has a handful of reviews at best, whereas if you look at dog coats rather than raincoats, these have a high level of reviews suggesting a much higher level of sales can be achieved.
Must admit I have never seen a dog in a raincoat - normal coat yes, but never a raincoat that I can recall - and we spend a lot of time hiking so do come accross a lot of dogs.