Because it's DS and that's just how it works? Many of the vaults and towers in DS can only be entered through the front door and that is if both (or all three or four) people HAVE the quest or have COMPLETED the quest. Believe me, it makes DS interesting.
Yes i know how you feel. It is especially hard when you only have a few friends online and if they haven't completed the quest yet or have the quest yet then they can't go in with you. It is not fun, but what can we do? It's supposed to be a challenge.
I didnt like the whole Cant teleport thing either with my first grandmaster because he was fire and almost every tower had fire people in it. but after a while I liked the challenge and learned how to do it solo. Main problem I ever have was losing connection within the for hours it took to do the ALL fire towers. But when i made a death guy it was much easier to solo and well i think they the best for DS but those are only to i have ever done so dont think i saying fire wasnt as good as death or ..... sorry of track ANYWAY after a while i realized it was because of an earlier complaint about lower lvls entering dragonspyre unfairly. i think this makes it to where those lvl 28's actually have to EARN the right to do ds. only place other than that is labyrinth and malistare instanced (great spyre,Dragons Maw, Dragon scale path, Malistares lair)