RSA, and to a somewhat similar extent Diffie-Hellman, bases its security on the difficulty of factoring large numbers into primes. While a scheme like AES can use all 2n numbers, in order to break RSA, you need to guess prime numbers.
As there are far fewer prime numbers and the factors can be better guessed, we need far larger prime numbers than other schemes like AES. So each additional bit doesn't double the difficulty of brute forcing it. If you want to venture into the math, you can check out the Wikipedia page on RSA.
While no one (we know of) has managed to break 1024, it is always important to stay ahead of the curve. And since brute force isn't exactly 2256 times more difficult,we pick higher primes. Who knows what future improvements in mathematics could break 1024-bit keys.