That's not why, it's really a simple reason. Samsung saw all the money Apple was making fleecing its customers on memory upgrades and decided it wanted in on that money train as well.
The question I have is, are most Samsung users willing to pay for it though? If I buy one I would pay for the 128GB version, but I wonder if others would pay for anything more than the base model, especially when they have been used to using SD cards, know that there are other Android OEMs out there who offer SD card expansion.
Apple users have paid for more storage for years so they will carry on. Also many of them are longtime, loyal users who will only use Apple devices so they don't have any choice. Samsung users may simply say, they will buy a HTC M9 instead or buy the G4 if it supports SD cards.