Lol it's to grab as much cash as they possibly can.
Firstly you can never buy what you want for the specific amount of points that can be bought at a time, so you are always left over with points that will never be used and whatever way you slice it that's you paying for something you will never use.
Second by removing the concept of a fixed currency, by removing an essential element that is common to almost everything we do, i.e the price in your local currency. You; a. remove the sense of value for the item you are paying. 1,000 points sounds like something that is worth a lot but if 1,000 points equates to £5.99 then chances are the thing you are buying is gonna be pretty shit, b. you are encouraged to purchase more because hard currency value is easier to track than some smacktard made up currency.
I don't buy the concept that it is easier either. Steam and PSN both provide downloads valued in the currency of the region. If these two can manage it then surely one of the biggest, richest software developers on the planet can do it as well.