We have not resorted back to localized units because most of them don't make nearly as much sense as does SI.
For example:
1 m = 10 dm = 100 cm = 1000 mm
See the pattern? On the other hand:
1 yard = 3 feet or 36 inches
1 foot = 12 inches
1 statute mile = 5,280 feet (very easy to remember and use in calculations)
1 nautical mile = 6,076.1 feet (like one of them isn't enough)
Same thing with measuring mass:
16 ounces = 1 pound
1 stone = 14 pounds
Oh come on... how did they decide on those numbers? By pulling them out of a hat?
With SI you know that you always work with 10^x.
Imperial units are so complex that obviously there is a tendency to transfer to SI. One other reason is that in some regions non SI units are being deliberately pushed out. If you force your school system to use only SI, children will not have that much opportunity to use and get to know the old one their parents grew with. By the time they have their own children the old units will be almost completely obsolete. In three or four generations no one would even remember them.
The problem with defining time is somewhat different than defining mass or length since you can decide on their value as you wish. Time is defined for you and you can't do much about it except to figure out how will you divide that long period to shorter one.
That longer period is defined by planetary rotation, also known as a day. One day on Earth lasts 24 hours, or should I say it was decided that 1/24 was going to be called an hour, since a term "day" is much older than a concept of "hours". There are different theories on why they decided to divide a day in 24 parts (12 for day time and 12 for night regardless on how long they actually last), but I hate them for it no matter what reason.
And now comes the fun part. There have been attempts to create various units of time that are easier to do math with than our current hours, minutes and seconds. For example Swatch Internet Time.
Swatch (the company that makes watches) tried to introduce decimal time. One day would have 1000 "beats", so when you ask somebody what time it is, you get an answer like 435, or 789.
I even have one of their watches capable of displaying it.
It was not the first attempt to introduce something like that, but it never happened. Mainly because SI unit for time is second and not "beat". You see? People are lazy to spend a day and recalculate couple of formulas.
So let's say that humankind shifted to this Swatch Internet Time, we would divide our day in 1000 parts and be happy. Please mind the fact that we are still just talking about Earth where one rotation (day) is always 24 hours long.
If you find yourself on some other planet whose natives decided to divide their day in 1000 parts as well. Splendid. We can tell each other time without any problems. Oh, wait.... no we can't. Their day happens to be 2.7557 times longer than ours, so their 1/1000 day does not match ours.
So should we reprogram our watches to show their time or should we force them to change theirs? Or maybe we can meet somewhere in the middle so none of us will be able to catch any sleep.
And what happens when a new world is discovered? Do we force them to conform with us? There are two of our planets after all. But what if they already met some other race and decided to share a standard time with them?
You can't simplify a calendar too much because sooner or later you'll start noticing the accumulation of that simplification, and you'll be either loosing days or end up having some extra.
Each planet must have its natural day. CallyArcale explained about circadian rhythms.
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