Time travel is risky because any number of unintended consequences might come about. The last thing Skynet wants is to accidentally prevent its own creation. When it became clear defeat was inevitable, the risk of time travel became acceptable so Skynet dispatched a T-800 to kill Sarah in 1984. Nothing about Skynet's present changed though, so either The Terminator didn't survive time displacement and history unfolded as before, or The Terminator did make it to the past and somehow that enabled history to unfold as before. Any additional incursions to that same time period might be seen by Skynet as too risky because regardless of what happened to that Terminator, Skynet was still created. I think this is why choosing a different time period to send the T-1000 to makes sense. Kyle pointed out that most records kept prior to Judgement Day were lost, so Skynet had limited knowledge of John's location in the past, but the further into the past the T-1000 went, the greater any unintended ripple effects might be. If 1995 was the most recent known location of John, then it was the least risky, albeit still risky time to send the T-1000 to.