I think it’s because when stories were set in the present day (or near future), they tended to be vague about the date so as to aid suspension of disbelief. While in stories set in the far future or past, a year was usually enough to give the viewers all the info necessary (and meant the writers didn’t need to worry about when any specific noteworthy events happened that might affect the story, if that wasn’t something they wanted to deal with).
I think it’s because when stories were set in the present day (or near future), they tended to be vague about the date so as to aid suspension of disbelief. While in stories set in the far future or past, a year was usually enough to give the viewers all the info necessary (and meant the writers didn’t need to worry about when any specific noteworthy events happened that might affect the story, if that wasn’t something they wanted to deal with).