why is temp. at which crystals first form recorded instead of the temp. at which they stop?

Actually you should record both. It is very difficult to record the temp where crystals stop forming (hard to say when something stops) but you can get that lower temp by going from solid to liquid and recording when the melt first appears. You record the temp where solids start to come out of a liquid because that is the saturation temperature. The two will not be the same except in a simple or "pure" system (one component).