It is not completely clear why Samsung decided to remove the SD card slot from their phones. But considering their main competitors stopped it a long time ago, it's no surprise that Samsung also jumped on the bandwagon.
Some speculate that the removal was due to the fact that SD cards are much slower than built-in internal storage. Therefore, in order to speed up the smartphone's functions, Samsung decided to remove it. Additionally, having an SD card slot places limitations on how sleek and thin you can make the smartphone, which could have also played a role in its removal.
It is also a good financial move. If you restrict customers' ability to upgrade their storage when they see fit, it means you can set the price for each storage level.