Here in Japan, Amazon is really popular, especially with expats. Other online sites are Japanese-only, so if you don't read Japanese well, Amazon is just much easier to use because it can be set for English, and will auto-translate everything that's in Japanese. The checkout process is also really simple. Japanese websites tend to have really poor UI too, and Amazon doesn't. Also, Amazon has relatively easy returns, unlike most Japanese retailers.
However, according to the grapevine things are getting worse. Instead of using reliable local shipping companies like Yamato or Sagawa, supposedly they're doing more delivery with freelance contractors and the results aren't good.
For example: I recently ordered a €600 printer at a local retailer. The printer shipped in an unusable state (it refused to print and one of the paper feeds spit out a metal spring at me, not a good sign), so I sent it back. It took 6 weeks and an angry email from me for them to ship the replacement unit. With Amazon this just doesn’t happen, or at least, has never happened to me.
I think Amazon is going back to being a place where you only get certain things when you know exactly what you're looking for.
Amazon search is so useless that about half the time I use DuckDuckGo to search Amazon itself.