CMUCL could have been ported to x86 earlier, I was running UNIX on a 386 in 1987 but only had AKCL for it. I was the first beta-tester for CMUCL on FreeBSD in 1996.
ports usually don't happen just so. If there wasn't one, I would interpret it such that there was little demand at that time.
SBCL later was forked from CMUCL to have a simpler implementation and build system. Recently the port to M1 Macs went smooth. The person(s) doing the port did an excellent job.
Porting CMUCL to x86 required someone having access to a machine that it already ran on as well as a 386 running UNIX, plus knowing that the CMUCL sources were available and having internet access to get hold of them. I think these were the limiting factors rather than lack of demand.