they died.
Is that canon? I'm just sayin', it seems highly unlikely that every single Picard in the galaxy except Jean-Luc's grandfather simultaneously took a vow of celibacy, leaving JL & Robert the absolute last hope for carrying on the family name. There must be a 2nd or 3rd or 52nd cousin living contemporaneously out there somewhere. Trust me - I know there are dozens of Bowdens in this country I have no chance of ever meeting, but with whom I share common DNA. Neither I nor my sister have children, but we are not by any means the "last" Bowdens, even if all our first cousins and their progeny were removed from the equation.
But then again, I may be missing vital information about the Great Picard Genocide of 2597 about which every Star Trek geek knows the name of every single Picard who was stood up against the wall.

It ain't that important - I simply thought "no more Picards" was just overly melodramatic.