Let's see what happens when we convert -0 to 2's complement.

Value of 0 in binary                             0

Pad out to 8 bits                         00000000

Flip the bits                             11111111
Add 1                                           +1
                                          --------
The answer                                00000000

In this case, the carry propagation never stops, but travels way behind the leftmost place. Since we can only store 8 bits, we throw away this 9th bit, and get 00000000. Thus, 0 has but one unique representation in 2's complement, and there is no separate -0 as there is in 1's complement.