Voldemort rose to power in 1997. He destroyed all records of muggle borns. Therefore, all muggle borns that would have been younger than 11 at the time of his takeover would never have gotten their letters because he's an evil wizard and destroyed our records. The Ministry is still in the process of finding all muggle borns.
HARRY POTTER FANS, YOUR LETTERS ARE STILL OUT THERE. HAVE FAITH!