You're assuming it was left out. We don't know if anyone ever suggested it. I think the two parties, the U.K. and China were content that the word be “elections” and to keep it vague, with the knowledge that these things would be fleshed out later on.
But of course, you have to know that in the drafting process of the basic law, the constitutional instrument, which is a Chinese statute, the British were not involved. It was a process that involved the drafting committee made up of mainland members and Hong Kong members, but mostly mainland members. In that process they arrived at the words “universal suffrage”.
Who proposed it? We don't know because we don't have the minutes from the meetings. They're not available. It was in a Chinese process, as the Chinese will claim, that those words came out.