If you ask me, Thorn is a popular choice for Trials because of the nature of the Elimination game type. Players are being more cautious, they won't often be engaging you openly unless they are forced to. So there is a lot of taking shots and ducking back into cover, the Thorn's DoT can still kill someone who has retreated back to cover. Now if Thorn had lower range, like TLW, then it might be a different story. But it hits just as hard at medium-long range as it does up close.