Its clear from Tanach that Hashem performed open miracles during those times. What is the reason that there are no open miracles nowadays (i.e. ones without any clear natural explanation)?
See Ramban Shemot 13, 16:
ובעבור כי הקב''ה לא יעשה אות ומופת בכל דור לעיני כל רשע או כופר, יצוה אותנו שנעשה תמיד זכרון ואות לאשר ראו עינינו, ונעתיק הדבר אל בנינו, ובניהם לבניהם, ובניהם לדור אחרון.
A free translation:
there is no need to make miracle at each generation for each miscreant and heretic. It is sufficient to remember the miracles of Egypt.
A miracle is a demonstration of Divine knowledge, Justice and intervention of G-d. When somewhat is demonstrated, you need not to demonstrate again and again. When Rishonim builded "scholastic Hebrew", they called demonstration "Mofet" for logic demonstration. You can learn from this what is a miracle. After the cancellation of many errors, no need to new demonstration.
@mevakesh wrote that
Ramban does nothing to answer the question.
@mevaqesh "formally speaking" is right. A = הקב"ה לא יעשה אות ומופת A => we need something else, C = we have Mitsvot. But something is not mefurash in Ramban (in Torah learning formal logic structure is not entirely literated). The world in its "natural" form is good, and is a will of G-d. And is good for non-miscreant persons. If a teacher makes all his teaching for a student that don' want hearing, he will cause a damage to others. So, the Ramban has something to do with the question. Mitsvot, particularly Mitsvat Tefilin (you know the text inserted in the black boxes but you are not reading this. I.E. you remember that it was demonstrated but you are dealing with something else) are here to remember demonstration, demonstrations with witnesses. Witnesses transmitted Masoret of mitsvot.
According to the Gemara, Berachot 20a, it's because we're not willing to sacrifice ourselves Al Kiddush Hashem quite as readily nowadays.
Rab Pappa asked Abbai : How were our predecessors different from us that miracles occurred for them but not for us? Is it a question of learning ? In the time of Rab Judah, their whole study was limited to the Order "Damages," whereas we study the six Orders; and when Rab Judah reached the paragraph in Mishnah 'Uksin : "If a woman presses vegetables in a pot" (some declare it was the paragraph : "Olives which are pressed with their leaves are clean"), he said, "We see here the conflicts of Rab and Samuel," whereas we study 'Uksin in thirteen lessons ! Yet, with respect to Rab Judah, when he took off one of his shoes, the rain descended ; but we afflict our souls and cry aloud and there is none that takes notice of us ! He replied : Our predecessors jeopardised their lives for the sanctification of the Name, but we do not.