Okie doke, I checked out what Mark Hanson had to say about fret markers - he's a mondo fingerstyle guitar teacher dude.
I don't knoiw if what he says is true, but here's basically what he says, along with some speculations of my own:
Steel string guitars have position markers on odd numbered frets - 3,5,7, and 9, until you reach a full octave, which is the 12th fret. It's just the way the steel stringers do it... just to let you know easily where you are on the fretboard.
On nylon strung jazz guitars though, the fret markers are there to replicate the white keys of a piano when you're playing on the first string. He says to imagine a fret marker on the first fret too. So if you're playing on the E string of a guitar, the white keys on a piano would be the following frets:
0 - 1 - 3 - 5 - 7 - 8 - 10 - 12 - 13 - 15.
E - F - G - A - B - C - D - E - F - G
And these jazz guitars have fret makers at 3, 5, 7, 10, 12, and often 15.
So maybe it's for aesthetic reasons why there isn't a fret marker on the 8th and 13th frets, so you don't have 2 fret makers right next to each other. Otherwise you got all the white keys on the piano marked!
So I guess the ukes follow in the steps of nylon strung jazz guitars.
See:
http://www.acousticguitarforum.com/forums/archive/index.php/t-4542.html