Not an excuse, but here's one possible explanation.

With the US Diesel being new, BMW probably did not expect any temperature sensors to be required, since temperature sensors are typically rather reliable and last a long time. From the postings I've seen, it's pretty clear the the temperature sensors were failing at a much greater rate than expected. This probably put them into a shortage position and left BMW with the choice of using the limited inventory to build and sell more vehicles, or to fix existing customers' vehicles promptly. I suspect they gave building and selling more vehicles priority.

In the meantime, either the vendor could not supply more sensors quickly for one reason or another, or BWM wanted a design or manufacturing change to fix whatever was causing them to fail so quickly. BMW would probably put a temperature sensor prone to failure in a new vehicle, if the alternative was to not manufacturer and sell the vehicle (knowing they probably would have to fix a lot of them under warranty later). But they might wait on replacing failed sensors in the already sold vehicles until the new, more reliable design sensor was available.

Yeah, I know doing something like that stinks, but having been in the industry a long time ago, I know how companies would not want to lose a vehicle sale because of not having a reliable temperature sensor.