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.