As to why Samsung could be making the switch, there are several reasons. One of those reasons is cost, where it has been rumored that the Qualcomm Snapdragon 875 will be more expensive than the Snapdragon 865, which means that Samsung either has to increase the price of its phones or be willing to take a smaller margin on the phones it sells.
Secondly, and this is just speculation, maybe Samsung is looking to take a page out of Apple’s books. Apple has been relying on their own chipsets for years to the point where they’ve developed it to be powerful enough to replace Intel’s processors on their computers. Perhaps Samsung is also hoping to achieve something similar.
I can confirm that Samsung won't use Snapdragon 865 again for their S21, instead I heard that Samsung could use Exynos 1000 for all S21 varrient (US, China included) https://t.co/XN99kJMRVe
— Your favourite Chun is back (@Boby25846908) July 7, 2020
That being said, Samsung’s Exynos chipsets have actually been getting a rather bad rap over the years, where those who own Exynos handsets have found that they don’t perform quite as well as the Snapdragon variants. Take this with a grain of salt for now, but we’ll probably have to wait quite a while before we get the official details.