The government was figuring out what was going on. Shortly after the movie ends, the better part of the Western seaboard is nuked into oblivion in a Heroic Sacrifice by the USA.Folks outside of the Bay Area were noticing the phone lines being rerouted, and weird phone calls from there. A few people got word out. The problem wasn't so easily solved, though - they were fighting the isolated groups of pods back for several years afterwards.
The alien pods grew best with some flammable chemicals not typically helpful with earth plantsThat's why Matthew's cutting down light fixtures caused the warehouse to explode.
The aliens are plant-based aliensThey rely on the pods to pollinate and they don't need sleep like most animals. They are emotionless because plants don't understand emotions
The Remake is a loose Sequel.Miles did warn people, but he was too late. Much like how while Night of the Living Dead (1968) ended on a high note, but further sequels suggested that the human race is slowly being overrun. The Pods are doing the same and he's trying to warn as many humans as possible. Hence his short lived cameo in the remake.
- Maybe the pod people did get beaten back in 1956 before they took over too many towns and the government covered it up. After 20 years of biding their time the pods tried again.