I haven't done any research on this but I suspect that usage predates "DSL," or at least was popular before the term DSL was widely known and used for it. I mostly see "little language" used in older sources, mid-90s or earlier.

It seems to be making a comeback though! I prefer it too. It's not really more descriptive than DSL but it's not much less, and is less jargony and just cuter.