What could have been the reasons for this? Of course i cant know, and i doubt most of those not working at Areanet know either, but i'm not really looking for a serious answer anyway:
my theories:
1. Social Experiment. Areanet is working with psychologists in order to research motivation and/or punishment tequniques. They gave a vision what some would consider "Paradise" (Open Beta / UAS) and then "denied" this paradise again. In order to reach it (face it, UAS is still in the game, it just takes 500h of holding the right mouse button to push it), you have to "earn" it by doing "mindless", "pointless" activities. IF they can get people to "grind" for paradise lost, maybe they can get them to do other things too; in a more real/meaningfull setting.
2. Server Load. The business model actually requires the players to NOT have an interest in playing this game while they have better things to do. After all, players who play 24/7 only use bandwidth. People bought the expansions to single player games which you can finish in less then 20h. You don't need 100h/week to "hook" someone.
Areanet includes "grind" in the retail version to force all those people to grind for 300h, realize that they haven't even unlocked 1/3 of all the superior runes for one proffesion, and leave in frustration. 1 month before the expansions, there will be UAS

If the pvp was TOO good, people will be playing 24/7 all the time ....
3. Blackmailing. Selling the "time to acquire gold / items / accounts" may very well generate more money then the actual games. And where there is money ... If there was one fun online game where there was no need to grind away to buy gold / items / leveled characters, it may set a dangerous precedent ...
Prepare to see Guild Wars accounts on ebay soon:
GW Account, 4 lvl 1 characters with starter gear

Post your own theories, real or unreal. Or just turn this thread into the usual grind/earn-PVE vs PVP holy war thread.