One explanation that might immediately come to mind is this: In biblical times, God selected one group of people to work with, as part of an overall plan of salvation, and so was concerned only with that group. Nowadays, however, he allows those who wish to do so to reject him because he wants everyone to believe in him freely.
This, however, might appear reasonable only because it is based on familiar claims. It doesn't take much to see just how weak an explanation it really is.
To begin with, why choose a particular group of people at all? Why not work with all of humanity? Are we to suppose that that would have been too much for God to handle? And given that he didn't care about the Chinese, Africans, native Americans, and everyone else worshiping other gods, why did it anger him so much whenever some Israelites strayed from the faith? It couldn't be because having a group of non-observant Israelites would prevent his plan of salvation from being carried out. After all, when the Messiah did come, most Jews rejected him anyway; his followers were found mainly among the gentiles.
And the situation today is even more incomprehensible. God has now revealed his plan to the entire world, and yet the majority still rebels against him. Why isn't he, like in Old Testament days, commanding that the followers of other religions – who might lead the faithful astray – be slaughtered? Or at the very least, why doesn't he make sure these people know that Christ really is the answer? Instead, he seems perfectly fine with letting them worship as they please. What happened to all that jealousy? Don't get me wrong: I'm glad he's mellowed out. But I can't help wondering why.