Uplinker
The real reason is not because there are still a few aircraft around that find it more convenient, but because all aircraft found it more convenient way back when the system was put in place. The technology has moved on, the procedures haven't. That's the way we've always done it may not be a great reason, but it's still the reason. If we make the change, the C152 can equip with a cheap VFR GPS or do the math for the variation. I can't imagine a situation where an A320 would only have compass for direction information while still having flight controls to use that information but in that case they could do the math as well.
The fact is, at some point we are going to make the change. Why not now?