Because it would make STF chars too appealing, leading to even less peasants on the battlefield, making it so much more frustrating for regular low level players.
How often do low level players who are slow get to meet other low level players on the battlefield?
They are pretty regularly picked off by cav at the spawn anyway.
And besides having access to a STF character won't make the battlefield any more frustrating for everyone, if the player sucks, they suck. A non- STF character given enough time will not only learn to become better (like the STF character would) but gain better items, or go over level 30.
STF does not put you into an overpowered position, nor should it be particularly appealing. Even if you had your pick of any/every +0 item it would not be overpowered. After all, it's simple math to say that more than half of the players who play are level 30 or higher.
1-29 = 30-31
Many people retire at 31, these players will spend about half of their time at 30. Many others have continued to grind and are upwards of lvl 35.