Why does motherboards have two IDE connectors?

Answer 1

Many current motherboards don't have any. In the early 1990s when IDE became the dominant drive interface, the IDE controller was an add-on card that provided a single connector, allowing for two drives. When CD-ROMs first appeared, they too required an add-on card, and there were several different brand-specific varieties. After the ATAPI specification was released, most CD-ROMs used IDE. This presented a problem when a user needed to add a second hard drive, but the CD-ROM was already using the second IDE connector. This occurred in the same time frame where motherboards began to include more and more peripheral connections. Including two IDE connectors on the motherboard allowed for four devices, solving the earlier dilemma.

It should be noted that before the early- to mid-1990s, just about every port and peripheral connection in a PC-compatible except the keyboard and floppy diskette controllers was provided by an add-in card. Electronics were not nearly as reliable as they are today, and nobody wanted to through away a $350 motherboard because of a bad serial port!

Going back even earlier, the big reason was $$$. Peripheral connections were very expensive, and not everyone needed them. Because the cost would have been so high, few computers would have been sold. I still have in my possession an add-on card called the Quadram Quadboard, along with its advertising brochure. This card provided one serial port, one printer port, and 384K of RAM for the original IBM PC. List price in 1982 was $995.