1) GW2 is suppose to take place hundreds of years in the future. All our beloved GW 1 characters would rightfully be dead.
2) GW2 is an entirely new and different game. Every GW player needs to re-learn the game mechanics and leveling new characters from scratch would teach us how the new system works.
3) Anet does not want GW2 to initially be flooded with high level characters. That would create a barrier for entry for those starting out in GW 2 who hadn't played the first game.
My proposal is for Ancestral Characters, a way to have our cake and eat it too

Addressing Problem #1: Temporal Anomaly
Hundreds of years into the future, Koss is dead, Devona is dead, and [insert favorite character's name] is--you guessed it, dead! But what is death to the heroes who closed the Door of Komalie, defeated Shiro the Betrayer, and slew a god? There is a place that exists for such legends, and it is called the Hall of Heroes, where the souls of those who have saved the world rest. In Tyria's bleakest hour, wouldn't the gods see fit to release these heroes of old so that they could bring salvation? A story of how the gods called upon ancient champions to assist mortals could be used to explain how GW 1 characters show up in Tyria hundreds of years after the events in Eye of the North.
Addressing Problem #2: Brave New World
Ok, so you have a level 20 character because you transfered your toon from GW 1, but you haven't the foggiest idea how to jump, slide, or hit a mob with your Ugly Stick. But wait... what if in order to transfer characters to GW 2 you must first commune with the gods and petition them for help! Anet could make sure GW 1 players go through all the proper tutorials and get their GW 2 training by requiring a native GW 2 character to reach a certain skill level before GW 1 characters can be transfered to that account.
Addressing Problem #3: No Pain, No Gain!
New Player: "Hey, how come you have a level 20 character just because you played GW 1, while I had to start my guy at level 1?"
Old Player: "Listen kid, before I could petition Balthazar for the aid of my ancestors, I had to prove I was worthy. I paid my dues, believe you me!"
The nearest great temple of Balthazar is at the edge of a volcano, guarded by scores of level 24 fire drakes, a level 35 Charr shaman boss, and the demon prince Menzies himself! Anet could require Herculean trials before allowing characters to be transfered from GW 1. If a native GW 2 character has to climb Mt. Doom to free his GW 1 ancestors, he himself would have to be high level, and by that time having access to level 20 characters would not be a big deal.
I am only guessing at the reasons why Anet does not plan to support character transfers from GW 1 to GW 2. However, whatever their reasons, I am sure they could be overcome. Please do not dismiss this request out of hand, this is at the top of the list of what your fans want!
Sincerely,
Chinh
a.k.a Arcadian Mandragon