First, if we're talking about more traditional-style guitars (Fender, Gibson etc.), that's just how they'd been in the first place. Many (most?) players don't want those more modern big jumbos typically found on metal axes, just for conservative reasons.
I think both "full" and "medium" jumbos have their advantages. What I like about mediums:
I find it easier to do barre chords, but can't really tell you why.
The greater the distance between string and fretboard, the lower you can press them down, which slightly detunes the note, similar to a small bend. I don't mind about this a lot, because I don't press hard.
While there's more resistance to bending thanks to more fretboard contact, my bends seem to have more precision. Maybe because there's more feedback under my finger, compared to big jumbos that make my finger float over the fretboard.
Long story short, bigger frets are better for fast metal playing, while mediums feel better to me for rock, blues, etc.
Some people also say the big ones tend to create more fret buzz, don't know if that's true, because I don't see why there should be a difference if string height above the frets is the same. My Charvel/Jackson (big jumbos) do have more fret buzz than my Epiphone/Gibson (medium) at same string height above the frets, and my Framus with the flattest frets does the least, but that might just as well be build quality. The Framus' frets really are too low for my taste though.
