Era mais barato e permitido pelo sistema mais barato em geral. A Wikipedia tem um artigo exaustivo sobre o 8088 :
The 8088 was targeted at economical systems by allowing the use of an 8-bit data path and 8-bit support and peripheral chips; complex circuit boards were still fairly cumbersome and expensive when it was released. The prefetch queue of the 8088 was shortened to four bytes, from the 8086's six bytes, and the prefetch algorithm was slightly modified to adapt to the narrower bus.
Em comparação, o 8086 possui um barramento de dados de 16 bits, o que significa que os chips do sistema que o acompanham precisavam ser 16 bits, o que os tornava mais complexos e caros.