Why do restaurants not carry Dr. Pepper?

[Rain Man]

Seriously, why?

I go into restaurants all the time and ask, "May I have a Dr. Pepper or Mr. Pibb?" and I get told, "We only have Coke, Diet Coke, and Sprite", or "Pepsi, Diet Pepsi, and Sierra Mist". Sometimes there's a Mountain Dew in there, but it's rare to get a Dr. Pepper or Mr. Pibb.

Why is this? What's so hard about having that fourth type of soda? Is it cheap to offer three types and really, really expensive to have a fourth? Does the existing equipment only have infrastructure for three types? Do health departments put you in a high-risk category if you offer four types? Why do fast food places offer four types or even more when sit-down restaurants can't seem to get past the magic number of three?

And why do they always have a soda, diet soda, and white soda? Do the soda companies offer packages that specifically exclude Dr. Pepper and Mr. Pibb? Why? Do they also offer packages of, say, Coke, Diet Coke, and Mr. Pibb, but no Sprite? Or maybe Diet Pepsi, Dr. Pepper, and Sierra Mist, but no Pepsi?

It's not a big deal to have a fourth soda, restaurants, and it damages my dining experience to not get Dr. Pepper or Mr. Pibb. Get on it.