It's just an awesome tuning, that's all. Remember we're talking about a much shorter scale length than guitar. In first position on a mandolin, the first finger covers the first two frets, the second finger handles frets two and three, the third finger takes frets five and six, and the fourth finger takes the seventh fret. So your three strongest fingers do most of the work. You can play an entire octave on two strings without shifting position. Plus, since all the strings are the same interval apart, it's pretty simple to move a lick to a different key, switch octaves, etc.
I played guitar for years before I started playing mandolin, but I find it a lot easier to figure things out on the mandolin, and I can improvise with a lot more facility on it. It's just more transparent. The difficult part for me is just the sheer physicality of the thing -- having to move twice as much metal around for each note. It'll shred your hands if you're not careful.
