Selection of class level?
 

Velious covers content for about thirty levels. Wouldn't it be in the server's interest to get some lower level stuff tested, rather than just occasional raid runs?

I want to make a 35 SK for example.

Is this a technical limitation or staff decision?

Well originally the devs wanted you using your live characters. So if your best character was level 35, you'd go test level 35 content. But people kinda complained until they added the buffbots.