Because When the game first released, the time to kill in crucible was a lot longer. Now auto rifles were very strong- because they were a fairly balanced system of damage and accuracy. Nowhere near the current 2 shot meta of thorn. It took a steady burst of bullets to kill a target. Thorns range stat is broken. It's accuracy is broken. You really can't compete against it. That's why there is so much hate.
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Thorn and TLW have always been two shots. Nobody used them at the start of the game because nobody had them. Xur has sold TLW 4 or 5 times, and pretty much anybody who wanted a thorn should have one by now.
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Well you can compete... with other good weapons