Let me introduce you to a term here - cost/benefit analysis. Putting in an SDCARD means making the space for it, installing the circuitry for it, and implementing the software to support it. Then of course it becomes another possible point of failure that requires customer and warranty support.
By putting in a non-removeable battery they can use what are smaller probably cheaper batteries. They eliminate the need for the user to open the device and potentially damage it in the process. Because the battery is smaller they can either possibly support a more desirable feature or simply save the weight.
Locking the bootloader prevents the average user from hosing the device that results in customer support calls.
And as I mentioned skinning the device is all about differentiating so stands out to make that sale.
Now granted all of this may only cost a few dollars per device, but when you add it up across millions of devices it starts affecting the bottom line which is the ultimate goal for the company. If their research shows they can save money by shaving these features and still sell well to their target market, why wouldn't they?
And this isn't about defending them but explaining why the market is going the way it is. The answer is always money. If the market says phones lacking these features are acceptable then you will see more of them. If the market says they really want phone with these features, then you'll see more phones with them.