Heh, you have just opened a can of worms with no real answer as to "why". But I'll offer some comments:
The .30-06 was the military round of choice for the USA for the first 2/3's of the 1900's. (M1903, M1, BAR, M1903A4, M1C/D, etc). Then something odd happened, the US military spent millions of dollars developing a new military round, which eventually became the .308 (7.62x51). Now, the 308 shoots the same bullet (.30 cal) as the -06 at less velocity, though it does fit in a short action. Now, the NATO adoption of the .308 has essentially doomed the -06 from any further military use.
In reality, the -06 is great, better even then the .308 and I'm a big fan of it. It shoots the same bullet a bit faster, though its not quite as effecient with powder burn as the 308. The .30-06 was the round of choice for sniping in the US military up until the adoption of the M40 and XM21 in about 1969.
hope that helps