Because I'm convinced that future Trunks is a complete idiot. Want a list of things that make Trunks a dumbass?
1) Interfering against Freeza, King Cold, and Freeza's men. Trunks was from the future... where he knew that Goku was going to show up and take them all out without any casualties. Trunks got there, looked around, and thought, "Durr, Goku's not here... I know! I'll do it!" So he didn't see anything wrong with the fact that in his dumb little mind, Goku was "late," and then he didn't think that anything would happen to the timeline now that he interfered where he wasn't supposed to and basically exposed himself to the Z Warriors. He sincerely didn't think that any of that would have a consequence. Then when Goku says he could have used his teleportation to get to Earth quicker, Trunks was so stunned that he interfered for no reason. Like dummy, you come from the future; what he ll do you think made Goku "late?" You know that he comes and kills Freeza... Well damn.
2) Allowing himself to be seen by the Z Warriors -- including his freaking parents. Yeah, how could that possibly mess anything up? It's a complete miracle of plot-convenience that he was ever conceived in the first place. In fact, Bulma's and Vegeta's entire motivations changed in this timeline than his since Vegeta was training for the cyborgs, which means that somehow, even though they were in totally different places in their lives than in Trunks's timeline, they still got together. What luck.
3) Telling Goku the truth about who he was. Look, I understand his motivations for warning Goku, about the cyborgs, but... did he have to get into specifics? Couldn't he just say, "Look dude, you see I'm a Super Saiyan; none of that's important. You know I'm telling the truth. I can't get into it right now, but these cyborgs are coming man! You gotta prepare for them! I'll tell you who I am one day, I promise!"? Did Goku really need to know who he was and where he came from?
4) Not taking a Namekian (Cargo perhaps) back into the future with him to become the new God of Earth. Let's face it: Dende leaving New Namek to come to Earth is no different than if another Namekian character came to a different time with Trunks. Just saying...
5) Being so quick to conclude that #19 and #20 are not the "right" cyborgs. Look, I know that I have no choice but to believe this as fact, but Trunks doesn't know anything except for how they look in his time. He was a baby when everything started and nobody would be around to tell the story. He doesn't know that there was no #19 and #20 before #17 and 18 were activated. It's not like #17 and 18 looked differently; they were activated hours after #19 and 20 appeared, so... how would he know if the timeline was going the way it was supposed to or not? My theory was always that Trunks was a little off on his time. He said that Goku died before the Z Warriors fought the 17 and 18... and had he not given Goku the medicine, that's what would have happened. Instead it became, "Goku got the virus later than expected and 17 and 18 showed up later than expected and 19 and 20 are completely new." Well... how would he know that? All he knew was that Goku died before 17 and 18 showed up, which is exactly what would have happened. But, since it was basically told to us by the writer, I have no choice but to follow it.
6) Believing that #16 didn't exist. I mean for real, how would he know? 16's entire motivation was to kill Goku. If Goku was already dead in Trunks's timeline, then 16 would probably just go away and sit on a cliff for eternity or something. There's zero reason for him to be so conclusive that 16 didn't exist in his timeline.
Trunks seems to be basing everything on something that his mother -- who wasn't even there -- told him and he was a baby during the time. If anything deviated from the story, he went, "Oh my god, that's different!!! How could it have changed?!?!" Um maybe it didn't. Maybe you just didn't know about it because there was nobody around to tell you? Meanwhile, he does things that jeopardize time, but doesn't think anything of it, but freaks out at other things.
Trunks is a total idiot and has to be the dumbest time traveler ever. Marty was a better time traveler than this goof.