For a causal linguistic reason, my guess is it's because sober is a fairly common word otherwise and is very easily pronounced, whereas teetotal is much weirder to say and has a pretty specific meaning, so one may go a long time without seeing or hearing it enough to really stick.

The etymology is also strange - it originally meant "T-total" abstinence from alcohol as in "total with a capital T". A funny word, we probably could have done better.