AES-256 is not the most secure variant of AES any longer, as it appears that variant with key extension improves certain types of attacks. One of those, called a related key attack, requires only 2^119 time against AES-256, and 2^172 time against AES-192. (Time against AES-128 is approximately 2^128, I'd wager about 2^126-127).

This is a very particular kind of attack, however, and it doesn't necessarily mean other weaknesses will be exposed. Cryptographers, though, are a conservative sort by nature, and many feel more comfortable sticking with AES-128, which was part of the original Rijndael specification and what was designed most rigorously. AES-128 also seems to have had the fewest weaknesses developed against it. The fact that any attack succeeded in reducing AES-256 to below AES-128's security can give crypto folks pause.

I think realistically they are all very safe and AES-128 is unlikely to be broken by anything within the next decade. After that? I wouldn't wager.

Please see the answer by cryptographer Thomas Pornin on AES 128 vs 256: http://security.stackexchange.com/questions/14068/why-most-p...

Because it's the most secure.