I think the most likely reason is that the producers figured Star Trek would be a distant memory by the 1990's, and in that case, who cares what is said to happen only 30 years from now (then).
So, ah, over on dallyink.com, they run a bunch of comic strips and a pretty sweet collection of vintage strips rerun from decades and decades ago. Their current run of Vintage
Flash Gordon is strips from late 1958/early 1959. I can't give you a link to show the whole story, but you can get the flavor from at least this one:
http://dailyink.com/shared_comics/d9bb7009-f6be-47d4-b695-4e7c102c9e48
In this story, Flash --- after having just encountered an alien spaceship zipping through the solar system and trying to dodge involvement in a galactic war that's been raging for tens of thousands of years --- has seen a major accident at one of the world's Weather Control Space Stations cause its weather-ruling energy beams to run out of control, sending hurricanes, blizzards, tidal waves, all sorts of chaos down to Earth. He grabs the nearest available ship and pilots it to a controlled crash just above the atomic truck mentioned in the above link near the only factory that makes the necessary parts. Now, obviously, the monorail is out and the roads are flooded and all planes are grounded and the nearest spaceport is three hundred miles away; and (in a subsequent strip) the electric beams of the automated highway are in none too good a shape either.
The setting for this world of super-science with a robust spaceport industry and weather-controlling satellite watching over the united Earth and working through the machinations of galactic politics? The far-off distant future year of --- be ready, please ---
Nineteen hundred and sixty-eight, nearly ten full years into the future.
My sole point (well, my main point; I also want to bring up to people that this is a pretty cool comic strip at least in its 1950s incarnation) being that while every artist wants to be remembered, they also know that nearly anything they write is going to be forgotten pretty fast. Setting stuff that's a decade to a couple decades into the future gives the thrill of letting the audience imagine being there to see this cool stuff, without it being so close that you can honestly expect anyone to remember this nonsense when the year comes to pass.