Why did G-d cause the salvation in Purim to happen in a natural way and and not as an open miracle as happened in Egypt or with Sanherib (Sennacherib)?
Had the miracle been open and wondrous, the nations of the world, specifically the enemies of the Jewish people, would say that miracles do not happen on a regular basis. The reason it happened at that time was because there were prophets and righteous people then. It was in their merit that the miracles took place. Nowadays, there are no prophets, and there aren't individuals who are on such a high spiritual level. Therefore, miracles would not occur for the Jewish people nowadays.
That is why G-d caused the salvation to happen in a natural way, in accordance with the norms of nature. Nature exists all the time, and does not depend on the merits of individuals. The enemies of the Jewish people must be concerned, therefore, that the Jewish nation's protection is not dependent on the merits of specific individuals, but is there all the time.
(See Ben Ish Hayil, Derush Geemal, Zakhor)
