Halakhah zu Könige II 18:4

This is certainly support to our words from the great scholar the Ridbaz. And we who can not compare to the greatness of the Rabbis in the previous generations do not be amazed at all that they did not arouse themselves to search for the chilazon. For we have found in the book of Kings (2,18) that the brass serpant that Moses made was destroyed for until that time the children of Israel were sacrificing unto it and called it Nechushtan. Regarding this it says in tractate Chullin ( 6b) Is it possible that Asa came and did not annihilate it, that Yehosafat came and did not annihilate it and yet all the other forms of idol worship in the world Asa and Yehosafat annihilated. Yet G-d did not arouse these great righteous men to annihilate it even though it was a great stumbling block to the Jews so that Ch----- could come and be on guard against it and it was hidden from them that it is permited to destroy the brass serpent as it is written in the Tosafot ibid 7. So too even more so it can be derived from a fortuori reasoning in our case when we are speaking only of the prevention of doing one positive mitzvah that we can say that it is from G-d that it be hidden from the giants of the previous generations the possibility of finding the chilazon in our time in order to leave place for the one who is ready to actualize this potentiality and to be on the guard regarding it.