The 68000 has a 16-bit bus but the 8088 is only 8 bit. I don't know about today, but RAM chips of the time supplied 1 bit per address, with the capacity being a square (since column and row are addressed using the same pins). So if you need twice as many bits per cycle, your options are basically to have twice as many chips (of a quarter the capacity each... probably cost-effective, but now your system has half the RAM), or twice as many chips (of the same capacity each... and now it's twice the cost).
Fitting twice as many chips on the board is probably a pain too. (And suppose you go for the 2 x quarter capacity option - now you need 4x as many if you want the same amount of RAM!)
I think around 1979-1980 that an Apple II with 16k was like $800. One with 48k was $1900. And they weren't passing any markup on the memory. Much different than today where the cost of DRAM is much smaller fraction of the cost.
I remember finding a scan of an invoice from the late 1960 for a Univac mainframe somewhere on the Internet. The total invoice was ~1.5 million dollars, the CPU costing ~500k, the RAM (~768 KB) costing 800k, and the rest ~200k.
Obvious reversal: the 8086 has a 16 bit bus, and the 68008 only 8.
There is a slower kid in every family. :)
Note that both the 8086 (1978) and 68000 (1979) were introduced ahead of, respectively, the 8088 (1979) and 68008 (1982). Basically these 8-bitsters were probably kind of a cost reduction following a familiar pattern in the hardware industry: product catches on, then customers want to put it into more and more things that are cheaper and cheaper, with simpler boards, where big MIPS aren't needed.
As the article points out at one point, at least some part of the reason to make the 8088 were existing peripheral devices that were not compatible with the 8086 I/O-wise.
(Those reasons are not mutually exclusive, of course.)
It was pretty easy to use 8080 peripheral chips with the 8086 and some very few clones did just that. IBM itself had to deal with the problem on the PC AT which had the same i/o chips but a 286 processor. It needed to replicate externally the circuit that the 8088 had internally due to the need to be compatible with the old 8 bit cards as well as the new ISA ones.
The 68000 would have been more of a problem since it moved from the matched memory and clock cycle scheme of the 6800 to a four clock cycle scheme with a complicated handshake. A special memory mode and two extra pins made it talk just fine to the 8 bit i/o chips. There was no need to wait for the 68008 for that.
One huge mistake that was made in the 8088 and 68008 (and I will suppose the TMS9980 as well, though I haven't checked) was that they didn't have a simple way to take advantage of page mode access in DRAMs like the original ARM did. If they had, the gap in performance compared to the 16 bit bus models would have been smaller.
Fitting twice as many chips on the board is probably a pain too. (And suppose you go for the 2 x quarter capacity option - now you need 4x as many if you want the same amount of RAM!)