Because AMD is losing ground when it comes to price/performance. Let's take AMD's latest and greatest, the FX-8150. It barely outperforms the Phenom II X4 980 BE. And where it does beat it, it's due to the program being programmed for multiple processors in mind (which cannot be done with every program).
Check it out: http://www.anandtech.com/bench/Product/434?vs=362
Versus the part it was supposed to compete against, the Core i7-2600K, it loses by a good margin in all benches except multithreaded ones that use more cores.http://www.anandtech.com/bench/Product/434?vs=287
And Maximum PC does place emphasis on price/performance when it comes to DIY stuff. AMD doesn't really have that anymore.