Because Cameron knew what Terminator 2's problem was heading in: John Connor. Since the savior of humanity was conceived on screen in a movie set in 1984, he would only be 7 years-old in 1991. Cameron realized that audiences wanted to see a Terminator sequel starring a 7 year-old about as much as they wanted to see a Star Wars prequel starring a 7 year-old.
Seriously, eat a dick kid.
His strategy was a ballsy sleight of hand, setting the film in a 1995 that looked exactly like 1991, which is to say, the most accurate rendering of the future in cinematic history. But he didn't go far enough. He arbitrarily stopped in 1995, giving us a 10 year old John Connor who, upon repeated viewing, is so obnoxious that you spend most of the movie wanting to see him shot in the face, fate of humanity be damned.
"If someone comes on to you with an attitude you say 'eat me.'" - The Savior of Humanity
Now let's imagine Cameron had moved Judgement Day to the year 2000, and set the film in the waning days of the millennium. Instead of Bart Simpson we get a 15 year-old John Connor. Casting director Mali Finn never has to fish for a male lead at a Pasadena Boys and Girls club, Edward Furlong never turns into a walking cautionary tale of childhood stardom and, more importantly for our purposes, Cameron never shits up a damn near timeless Terminator with botched attempts at taking the pulse of early 90s youth culture. Instead he would have given us a John Connor who, like his best movie, is too far into the future to touch any cultural touchstones, but still wouldn't have been too distant to listen to a kick ass Guns N Roses song.
We can't say who Cameron would have cast had he set T2 four more years into the future, but some pretty intriguing actors were the right age in 1991. Christian Bale was 16 at the time, clearing him to play a post-pubescent John Connor in our hypothetical 1999 T2. If Newsies era Bale wasn't striking his fancy, Cameron always had the option of a 15-year-old Leonardo DiCaprio circa What's Eating Gilbert Grape?.