according to the source, the issue is with the chip size: One problem with HBM is especially an issue for large GPUs. High-end graphics chips have, in the past

according to the source, the issue is with the chip size: One problem with HBM is especially an issue for large GPUs. High-end graphics chips have, in the past, pushed the boundaries of possible chip sizes right up to the edges of the reticle used in photolithography. Since HBM requires an interposer chip that's larger than the GPU alone, it could impose a size limitation on graphics processors. When asked about this issue, Macri noted that the fabrication of larger-than-reticle interposers might be possible using multiple exposures, but he acknowledged that doing so could become cost-prohibitive. Fiji will more than likely sit on a single-exposure-sized interposer, and it will probably pack a rich complement of GPU logic given the die size savings HBM offers. Still, with HBM, the size limits are not what they once were.