I think human ignorance.
You take a GSD six month old puppy. If it started barking and growling and nipping at it's owner, the owner would realize he/she has a problem and take steps to correct the behavior. They would take the behavior seriously.
You take a Chi six month old puppy. It exhibits the exact same behavior as I mentioned. The owner thinks it's funny. "Look how brave my little boy is! He thinks he's a big dog!" and the owner will continue to encourage this behavior by showing off to all of his/her friends the behavior.
Yes. I have never had little dogs, but my husband grew up with chihuahuas. He has a soft spot for the little guys so we added a chihuahua/dachshund mix to the family (a rescue, not a "designer" dog). We brought her home at 12 weeks and raised her the same way we've raised our other dogs. She's now 7.5lbs and will be 3 years old in April, and she's every bit as much of a dog as the others are. She's friendly with other people and dogs, very social, obedient, and athletic. She takes a 5 mile off-leash hike in stride and then comes home and snuggles with you. Never once has she growled at or bitten any one.