Frankly, because it's not Master of Orion or Alpha Centauri. You can have your Next War mod and your clone armies and walking battlemechs and aliens, but leave me the main game, please.
I'm inclined to see the optimal solution here as a variant on something that's been suggested elsewhere about picking your victory condition depending on era, so that there could be something like:
Medieval Era: military victory, culture victory, economic victory, achieve the Renaissance as "tech victory", diplomatic victory through the Apostolic Palace.
Renaissance Era: military victory, culture victory, economic victory, achive the Industrial Revolution as "tech victory", diplomatic victory through the Apostolic Palace.
Modern (first half of 20th Century) Era*: military victory, culture victory, economic victory, achieve the Manhattan Project and/or the Apollo Program as "tech victory", diplomatic victory through the UN.
Near-Future Era: military victory, culture victory, economic victory, starship as "tech victory", diplomatic victory through the UN.
with different thresholds for an era-specific culture or economic victory, and a large complicated project for the tech victory at each point depending on where you choose to end the game.
I would be perfectly fine with the Space Race landing a man on Mars and bringing him back alive, and no silly futuristic junk.
I think everybody's line for silly futuristic junk is different, though. There are any number of plausible real tech advances that could happen in the next five or ten years, well before we actually put a human on Mars if we do.
I would remind you about the time period that Sid Meier picked when he designed this game: six thousand years in the past to a few decades in the future. That wasn't an accident. It's very clear where the inspiration for the game comes from.
I remain unconvinced that it is necessary for the game to remain bounded to the original vision in that way when it has been expanded from that original design in so many other ways.
*I would ideally like to see the first half of the 20th Century represented as the Modern Era and the second half as the Space Age, such that we are now living in the very start of the next Era on again, but that's a different argument.