Because gameification removes morality from the system. It completely nukes it.
I was at a talk from one of the developers of 'spec ops the line' a few years ago at GDC, who explained this pretty well. He make a very compelling argument for not rewarding users (in that case gamers, but same difference) for their actions. Take for example the following situation;
There is a beggar on the street. He needs money. You just got the exact amount of money to buy something you really want. Giving the beggar the money means you can no longer but that item, but you don't really need it in the first place.
At this point, this is a moral decision to either keep the money (I worked for it, I really want it, I don't even know the guy) versus giving it to the beggar (He needs the money to survive).
When you get a reward for giving the money away (say, karma points that give you advantages), the choice is no longer as what you feel is right but something to be optimised. The choice would be; Give money, recieve points, get other cool thing. So your decision making would be; 'Is karma reward better then money reward?' rather than 'Should i help this person'. It's no longer about helping, its about a transaction.
If you want people to help others, this transaction system might work. But it's not without its downsides.