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
