I think it's just "white flight" to the suburbs with demographic changes in the Bronx. Jewish communities upped and left cities much more quickly than Catholics because it's easy for a synagogue to move while a church is anchored in a geographic parish. Also, moving to the Bronx was once seen as moving up from the slums of Manhattan to a nicer urban neighborhood, so it wasn't so hard to move again. And the older generation died off or moved to Florida.
http://www.thejewishweek.com/news/new_york/bronx_it%27s_desolation_row
I never knew the Bronx had a big jewish population. I haven't been to The Bronx in 15 years even though I live in Central Jersey. Its just a place you want to avoid. I'm just shocked people wanted to move to The Bronx at one point.
I think maybe all the Jewish people that lived in The Bronx maybe moved to Brooklyn, Queens(look at Anthony Weiner's old congressional district), or Long Island. I know a guy that relieves at work from time to time and he went to High School in the 50's or 60's in Long Island he said everybody was Jewish that was around him. A few italians were living in Long Island at that time he said too. Now I think the Hispanics are moving into Long Island with the bastion of Italians that are already there.
People who live in the Bronx tend to move to suburbs in Westchester and points north or, in some cases, North Jersey. Most don't move to Long Island or Brooklyn - the major Bronx roads and transit links point north, not east. I'd bet a lot of the successful children of the Jewish residents on the Grand Concourse moved to places like Scarsdale and Rockland County when searching for greener pastures in the NYC metro. As brittain33 said, some of the older folks died off or moved to Florida as the old Jewish neighborhoods turned more and more Hispanic.
Even today, there still is a sizeable Jewish community in the Riverdale section of the Northwest Bronx, along the Hudson River.
