Because, as I read the article, each key[i=0] represents a single value chain and you're going to have many chains in the store, presumably being written by mul

Because, as I read the article, each key[i=0] represents a single value chain and you're going to have many chains in the store, presumably being written by multiple processes. So if you were inserting based on some global incrementing counter, you couldn't be sure that key[n+1] belonged to the same chain as key[n]. Using iterated SHA1 gives reasonable assurance that key[i+1] will be unique while at the same time making it a function of key[i] so that it can easily be found given that initial key.