I think Venom was sucessful owing to the simple visual elegance of him being an "Evil Spider-Man." Those who were reading the strip when he debuted enjoyed the idea that the alien costume storyline paid off with a truly nasty villain, while those who came upon him later saw him with the same interest Professor Zoom or Bizarro evoked in earlier readers. Having the hero go up against an evil version of themselves is a long-standing comic-book storytelling trope.* That he has backstory with Spidey helps a great deal as well. **

* As is having them go up against an identical, earlier, later, or gender-reversed version of themselves...
** Professor Zoom has interesting backstory as well... I think about five different ones at this point...