Because time is not a place you can go to. Time is a measure how how fast the various reactions, and forces, and processes that make up your atoms, moves.
Those reactions can move faster, or slower. But there is no place you can go to to travel backward in time. Time is NOT a river or a line. Time is just a measure of how fast your reactions processed relative to the reactions of other things.
The reason time is sometimes talked about as a line is that it works mathematically. And the mathematics works if you reverse it too. But that doesn't mean the math has a physical reality.
When I said "I've never really understood why we shouldn't be able to travel backwards in time." I was referring to the statement from the article: "Ultimately, scientists may find that only travel into the future is possible, as the laws of nature may make travel to the past impossible so the relationship between cause and effect is maintained." particularly the part about cause and effect.
Technical matters of relativity aside, consider that you can't actually change how fast you move through time. You can't make time move faster or slower, at least within your own frame of reference. Time doesn't just move in a fixed direction, it moves at a fixed rate. Note that this is pretty close to how special relativity actually works, modulo semantics of "rate" when dealing with time.
There are also thermodynamic considerations. One direction of time does seem to be privileged in terms of entropy. This doesn't necessarily mean that time travel is impossible, as far as I'm aware, but it does mean that "forward" in time isn't completely arbitrary, and reversing that may be thermodynamically unfavored.
Most sci-fi movies and shows seem to represent time as a line. I think of our history as a line, but every step along that history had an infinite number of possibilities it could have gone. To travel back in time would be to move back along the timeline that our reality exists - but once at that point, simply by being there, you would never be able to move forward into the timeline you came from. You're now on a new timeline, and anything you do has no effect on the timeline you came from.
Also, I don't know about these forward "time travel" methods described in the article. By those definitions we are all time traveling right now - we are moving forward in time. Its just the idea that some people move at different speeds so that when you slow catch up to the speed of everyone else, you're now "in the future". Really you were always there, you were just moving at a different speed.