The 68000 was a high-end product, the 8088 was a lot cheaper, in a big part because of those design decisions, like having a 16 bit memory bus.

This design allowed for a smaller chip, and keeping backwards compatibility with the 8080.

But there is more: IBM basically stole the entire CP/M software ecosystem by using the 8088: assembly language CP/M programs could be more or less just recompiled for MS-DOS.

Yet, it extended CP/M by allowing you to use more than 64 KB vs. 8080/Z80.