Why 8086 was not use IBM PC?

Because IBM wanted to build a computer with an 8-bit data bus. The 8086 and 8088 are the same processor, with the 8086 running on a 16-bit data bus, and the 8088 running on an 8-bit data bus. This allowed IBM to make the most use of older designs that supported 8-bit data buses, such as the 8080, the 8085, and the Z80.