It had never been below freezing that long since we started recording temperature for large parts of Texas. Thus, home designs often don't bother dealing with the question "but what if its freezing cold for days on end?", so there's no consideration to putting a water line on an exterior wall. We even have tankless water heaters mounted in or on the exterior wall with plumbing only protected by a metal plate if even that.

In places where it does regularly get below freezing for long stretches of time they do try and avoid putting water lines on exterior walls specifically for this reason.

Poor construction of pipes in exterior walls with degraded insulation.