なぜ機械式ポンプよりエジェクター式ポンプを使用した方が良いのでしょうか？

The ejector technology has no moving parts thus  with virtually no maintenance, it will provide higher reliability. It is also much smaller in size and weight that gives you the possibility to place it closer to the point of suction or where you actually need the vacuum.  The mechanical pump also has a much higher noise level and produces heat, meaning that normally you would need to have it in an insulated room.