Many on this board answer this question in one of two ways:

1) It isn't hard to get, you just don't know what to ask for/or didn't do your homework. Personally I think this is a cop-out, even if there is a kernel of truth.

or

2) CO is always sold out, so it is only natural. I think this a practical answer but not necessarily a great one.

If you ask me, the answer is that CO miles (along with DL) miles are the easiest to earn. The 50/100/125 bonus miles for elites are pretty generous, and Plat is probably easier to earn than many other airlines' top tier. You also earn these bonuses on all ST carriers, which is not the case for *A, at least.

Finally, does it shock anyone that two of the carriers most often criticized for poor availability (CO & DL) are the two major US carriers allied with Amex? I mean, who doesn't earn a boatload of Amex points in some way? A business traveler making one Int'l trip a month would get 100k+ points without breaking a sweat. Even an infrequent traveler who uses their Amex for day-to-day spending could hit 25k pretty easily.