Since you can see past challenges back to 2008, you can see the evolution in the quality of the winners. Its is fascinating. I would explain it this way. First, iPhone photography capabilities increased quite a lot with each generation. Also, I guess the popularity of the web site grew over the years, increasing the number of submissions. Finally, the popularity of the iPhone makes it available in many more hands with photography abilities. Make no mistake, you will find winning pictures taken with older iPhone models. This year’s top photos came from an iPhone 7 Plus and an iPhone SE.
In order to submit pictures, you have to pay a small amount of money. It could explain why you won’t find any ads on this web site, which is a good thing. Over the years, they added categories which helps a lot the browsing experience.
If you plan to submit pictures, make sure you read their FAQ. I may try it for next year challenge.