The first 386 PC didn't arrive until September 1986. It wasn't until like 1991 that sales of 386/486 PCs went above 50%.

Most PCs in the wild were still 8088s or 286es at best.

DOS just couldn't really take advantage of a 386, so for most people, that 386 wasn't exactly useful until windows 3.0 was released in 1990. DOS extenders didn't really become mainstream until about the same time.