Coincidentally, I found this explanation reading through Microprocessor Interfacing Techniques 3rd Ed 1979 by Zaks and Lesea page 16:
The Standard Microprocessor System
Throughout this book, reference will be made to a “standard microprocessor.” The “standard” microprocessor today is the 8-bit microprocessor. Examples are the Intel 8080, 8085, the Zilog Z-80, the Motorola 8600, the Signetics 2650, etc. In view of the pin number limitation on DIPs (dual-in-line packages), the 8-bit microprocessor has become the norm. The reason is simple:
The Number of pins is limited to 40 (or 42) by economic considerations. Industrial testers required to test components having more than 40 pins are either not available, or would be extremely expensive. All standard testers will accept only up to 40 or 42 pins. In addition, naturally, the cost of the package itself increases rapidly over 40 pins.
