Your question was why it isnt used, so....
Why it ISN'T used in a lot of long range shooting is mostly because of the bullet selection. 30 cal is a lower BC bullet and people generally go with a caliber shooting 7mm or less so, the 6.5 cm for extreme long ranges. No doubt, the 30-06 is a great cartridge but gets outclasses by other rounds that shoot inside it with much less effort. Even in short action, the 7's and 6.5's are doing well, take for example the 7-08, if the 30 cal bullet had good bc, I don't think that would have been born because 30 cal have a gigantic selection of bullets compared to 7mm or 6.5's. I'm running 6mm cartridge for my short action and .284 in my non mag long action
Food for thought