Same reason we have crushed hats, we don't spend time worrying about appearances 
החיצוניות מעורר הפנימיות.
Just curious, as to why Lubavitchers wear their shirts untucked?

Always had that question, in a way it's a good thing that way you can tell who's lubavitch from a mile away as they have there shirts out together with there tzizzus and pinched hats, whether in a airport or looking for a shul or anything that chabad offers you can spot them like a sore thumb.
Great ppl ,they are there for everyone where ever in the world you may be, with countless stories with personal encounters אהבת ישראל