Do you know why people download the game and don't stay or are you just guessing? Because all the people that downloaded the game and stayed during the last 20 years didn't have training missions.
Try for a moment to forget everything you know about AH. Then imagine that you have played 100+ other games, half of them online, a dozen of them mmos. You got broadband, a 1 GB dowload takes less than a minute.
Now you are a typical new player.
Think how you would learn about the game. Probably by dumb luck, because there's practically no reviews on news sites nor any let's play videos. So most of the information you are going to consume is from the steam page. Or possibly the HTC website.
How much time do you spend on doing any reading before hitting the dowload-now-for-free button? Consider the time you spend reading vs the time it would take to try with just 1GB to dowload.
Now after reading the website for 2 minutes (and continuing to read for another minute while the download is running) you log in to the game. First impression, text look blurred (pixelated fonts on the mapped texture of the clipboard). Offline... training... online... let's go online and check the game. Arenas... 100 there, others empty, ok, that one. Oh, not really free, it wants money regularly. Well, let's try while it's free.
Ugly video, go away, ah, thete's a checkbox to get rid of them.
Now how to get a plane... weird interface. Let's ask someone... no chat bar. Enter doesn't do anything. I'm not too stupid I can open the chat bar in every other game.
Oh well, blurred text, wants money, cannot fly. Not worth spending any further effort because it's probably not good enough for 15$/month anyway. Website looked ancient, let's try something else.