My guess is that this is two-fold. From the 3rd-Party Publisher point of view, trying to reconcile your game's DLC sales via MS Points is a nightmare when all you care about is the actual dollar value. MS Reports daily sales for your items in MS Points. Dumb. It is actually even more complicated than that, but the gritty details I'll spare you. Publishers have to wait till the end of the month to get their restatement that then reports how on how much actual money the daily reports generated. These reports used to come in by e-mail as well, but luckily MS finally got a feed for them running, but still reports MS Points (kinda).
The second reason would be, again my guess, that not only do MS Points have a different conversion value in different regions, but DLC costs different amount of points in different regions.
In the US: 80 MS Points = $1
DLC for GTA4: 800 MS Points
In the UK: 111 MS Points = 1 British Pound
DLC for GTA4: 659 MS Points
Now my numbers aren't completely accurate because I'm too lazy to actually look the correct conversions up, but you get the idea. Trying to figure out how much money you made when all you are giving is points and what region the item sold in is a nightmare. Not only that, but MS doesn't take their cut from the MS Points, but rather from the final dollar value.
This isn't just one more hoop to jump through.
The hoop is one fire... and on the other side is a shark-tank... who have laser beams.