When Aflatoxin appeared to beta test, Malignant Cell was a 0/1 at first, after a few days of testing it was nerfed to 1/1 cos of the ability to fill opponents field in 4 turns making them unable to play more creatures, doing so meant that the opponents was completely blocked, and a few healing on your side made you win the game.
Turning it a 1/1 means if you want to fulfill enemies battle field, you need almost a shield to hold control, or die instead.
That's why it can't become 0/1 again.