here are the theories
1. Age/experiance- This is the one I have the most support from things I could find from the show
This is in my opinion the most reasonable and most likely reason. Notice peter is the oldest werewolf we've seen on teen wolf.
Now who's the next oldest werewolf- Deucalion? If you look at his werewolf forms his is very similar to Peters but smaller without as much hair. Then We had the alpha pack so why were they stronger then Derek when he was an alpha- 2 reasons 1. They killed their packs and emissary. they also has alot of members or 2.They are older then Derek . Part 1 of that lead me to one of my next arguments. So why is Scott such a weak alpha because he's MUCH younger then any alpha we've seen up to this point we should also take into account Scott hasn't been an Alpha long.
2. Maybe you have to kill a shifter (someone who can turn from wolf to human to werewolf) like how Peter killed Laura.
3. After seeing season 3 that a werewolf gets stronger killing other wolves maybe one simply has to kill enough of them to become this beast form. Like we saw with the Alpha pack killing other wolves makes your stronger we also saw this happen when Derek was forced to kill Boyd.