Well, it would be easy to keep the other quests open rather than lock you out, but the locking out is done for balance purposes. There's a limited amount of gear you can find in each of the quests, but if you had access to all three, the scrolls of enchantment etc. would add up.
I think that was one of the problems with CLIVAN, where you had mutiple new areas full of loot, all freely accessible, and you were severely overpowered in the end game. I won CLIVAN with all gear made of valpurium or GEF, enchanted to +6 at the least, stats through the roof and unlike vanilla IVAN, it wasn't hard or lucky to get to that point. I just had to explore everywhere.