It's because the overall shape of the passenger bus hasn't as changed as much as cars and pickups. You see the same with the school bus and other utility vehicles. Hardly anyone would confuse a car manufactured in the 2000s with one from the 1960s, but if you look at a flat-front Blue Bird bus, you have to look a lot closer to see differences.

Aircraft have the same thing. The Boeing 707 first flew in 1957 but doesn't look a whole lot different overall from an Airbus 318 that first flew in 2002.