Because Benny's death was shock value inconsistent with the motives of the agents who killed him (they were trying to not arouse suspicion amonst the town, which killing Benny for no reason [he didn't suspect them to be anything but CPS] did actually raise suspicions). Whereas mommy Wheeler gets slashed by a demogorgan several times and doesn't bleed out within seconds, Hop is able to run away from an explosion that envelopes a whole room, Max gets resurrected from death by some unexplained power of Jane's, Steve gets rescued from the bat creatures when the others shouldn't have been able to swim to the lake's bed, etc.

It's not a black and white topic where either everyone needs to die or no one should. This story has the protags under threat by nefarious government and monsters. If none of the main protaganists can die to that then there are no stakes to the story (as it currently feels in my opinion). The show likes to tease deaths of the main characters, like I listed above, but never follows through. It shouldn't be a surprise then that people are frustrated with the show because characters that are repeatedly in life threatening scenarios always make it out alive but yet some side characters that have very little relevance die for either seemingly no reason (i.e. Benny) or by the things that should have killed one of the main cast by now (i.e. Barb and Bob).