A few thoughts;
1) Virtually every grocery store sells bagels, lox and cream cheese - what was once an exotic staple of the Jewish deli is now a common staple everywhere.
2) While it is true that Jews have been unusually successful in America, running a deli isn't likely to pull in the same amount of money as lawyering, doctoring and sciencing. A lot of those delis were run by men whose kids went to college and decided they could do better than taking over the old man's business. Good for them.
3) There is less anti-semitism now than there used to be. It is possible that Jews don't seek community at Jewish deli's as much as they did in earlier generations.
4) Perhaps Jews don't live as close to Jewish delis as they did when they