Its about profit margins. The iPad Pro does not need much more ram for its display. I've read somewhere that the difference in screen size only requires about 5 percent more ram. The reason Apple skimps on these things is to increase profit margins. Same reason we still have a 16GB iPhone, same reason the iPhone 6 had 1GB of ram etc it goes on.
The difference in cost from 2gb to 4gb for a company like Apple is pennies. Cost is not the issue here.