It's not about the size of the stacks IMO, the Bluetooth stack is notoriously difficult to interact with since the implementation varies wildly across different platforms and maybe architectures, and communicating with those different stacks is difficult, in a way. And within each stack you may find many features that are sparsely or not documented at all.

Just saying, but if it were easy, cross-platform libraries would have been developed long ago, but to this day most libraries have implementations where some functions are either finicky or not implemented. In no way am I trying to criticize the authors of those libraries, IMO it is a commendable achievement that they have understood the standard and the different stacks and have attempted make it cross-platform friendly and easy-to-use, but that's just the state of Bluetooth these days.