The problem with Forretress is that once it's out (as a lead), your opponent knows exactly what it's going to do. They'll know it has Stealth Rock/Spikes and Explosion, so it's not like you're really going to surprise any switch-in (probably a ghost) with Explosion. In addition, your opponent is going to be using their lead to set up their own strategy quickly, whether it's through setting up their own entry hazards, Taunting Forretress (remember, a lot of leads have Sash, meaning they'll survive Explosion), putting Forretress to sleep, or just plain killing Forretress. You don't want a Pokemon with speed that poor as a lead, particularly when other, less predictable Pokemon can do the same thing faster and with much better offensive power.
In general, you're going to be picking and choosing entry hazards on a single Pokemon, because you need to maintain momentum. If all a Pokemon can do is set up entry hazards and (predictably) kill itself, you're going to have a lot of trouble when your opponent switches in something that can easily stop that strategy, and in the process, you'll be taking damage from their entry hazards. Fortunately, Forretress has good moves for when it's outside of the lead position. Since two of its best moves, Explosion and Rapid Spin, are useless against ghosts, you can use Payback to hit them on the switch-in, or use Gyro Ball to hit faster Pokemon (and fortunately, Gengar fast AND a ghost).
It may seem cool that Forretress can set up a bunch of different entry hazards, and God knows, I love using the Spikes/Toxic Spikes/Stealth Rock/Payback Forretress as a lead in the Battle Factory. But in battles against human players, it needs to be able to do more than one thing, or it will really drag your team down, and probably get itself killed before it even gets to set up whatever entry hazard you want to use the most. By giving it multiple options (and only one or two entry hazards), it becomes a much more useful Pokemon, keeping your own field free of entry hazards, scoring KOs against ghosts, buying itself more time to set up your own entry hazards, and exploding once it's served its purpose.
