Originally, the minimum length for a segment was much shorter.  After a while, they realized that short segments were problematic because the accuracy of GPS units and recording wasn't good enough to accurately track times over short distances.  With short segments, there are only a few data points and if one or two of those are a little off, it makes the results totally wrong.  So, eventually, they changed the minimum length of a segment that you could create, but they left the short ones that were already in the system which is why you will still see some that are shorter than anything you could create today.

It's a bummer.  I have an extremely steep 200m climb but can't create a run segment on it.  

If a segment is private, it shouldn't have these restrictions, as it's for reference only.

GPS is much more accurate these days, especially if you have multiband GPS. When I first started using Strava, my GPS devices gave very crude GPS tracks, especially when in forests etc whereas now I get very clean and precise tracks. Why are we still setting standards based on old GPS tech? Also why does Strava only look at the start and end points of the track and nothing in between? I often ride at bike parks where there might be 5 or 6 trails all starting at the same point and ending at the same point but they are distinct trails and yet Strava cannot determine which one I was on. On the map it even shows the trails and my GPS line almost perfectly lines up with the trail and yet Strava still thinks I did the trail beside it which does not line up with my GPS track at all, except for the start and end points. I can't wait for another company to come along and Show Strava how it should be done. They are resting on their laurels.