I'm convinced that the "prevailing technology" was what was used, and most 40-pin devices in the '70's were in the 600-mil DIL. It was tricky, back then, and remains so, to some extent, to integrate EPROM and SRAM with high-density logic. Today it's SRAM and FLASH, but the problems are still the same. For buffers and multi-word logic, I suppose they can integrate stuff into small enough dice that the lead frame technology allows 'em to use pretty small packages, but I've no idea what problems having high-density logic, high-density SRAM, and high-density FLASH on one die presents. Clearly it's not easy, else there'd be more SRAM on 805x dice, among others.

RE