Because it works very well on the game that most people will hunt on a regular basis. There is a very good range of bullet weights and quality ammo is made by everyone that makes ammo. It is easy to handload for and if you don't handload you can get ammo just about anywhere. I've seen 30-06 ammo on the shelves in Cameroon, Zimbabwe and South Africa and I can't say that for the various short mags.

I'd happily hunt 99% of the game in the world with a 30-06. The only game I'd hesitate to go after is the really big stuff like elephant, rhino and cape buffalo and if the 30-06 was all I had and I could get quality 220 gr FMJ's I'd hunt them as well with the 30-06. The only cartridges I can think of that is better for all around hunting than the 30-06 are the 9.3x62 Mauser and the 375 H&H Mag. One thing all 3 calibers have in common is they are all over 100 years old and they are all commonly used. I'll admit the 9.3x62 isn't that common in North America but it is very popular in Europe and Africa.

Remember, the game itself hasn't changed. What worked well 100 years ago will still work today. That being said, I personally don't have a 30-06 since I prefer the 7mm Mag due to the fact I normally hunt wide open spaces. But if I was only going to have 1 gun to hunt with, I'd look long and hard at the 30-06.