One of the most common things hated about Teen Wolf is how Scott McCall is so weak as an Alpha compared to all of the other Alphas but they don't actually realize why he is so weak compared to them. This post is a detailed explanation on why he is so weak compared to them.
Throughout Season 3a we are hinted to how Scott will be an Alpha. Firstly by Braeden in the hospital. When brought into the hospital, after saving Issac from the Alpha Pack and after being chased by Ethan and Aiden, she tells Melissa that she is looking for alpha. When Melissa asked why she is looking for Derek, Braeden is wheeled away and says "No, not Derek....Scott...Scott McCall". This was our first hint.
Another hint was when Scott tried to pass the Mountain Ash to save Doctor Deaton from dying. When this happened his eyes turned red and when Alan saw them he was shocked and pleased with it as we learn just after Sheriff Stilinski shoot the rope dropping Alan to the floor, that there is such thing is a True Alpha (Someone who doesn't have to kill to get it, but someone who becomes it due to pure will). We also learn that Deaton has been hoping that Scott would become one. There are also many hints throughout the season.
When he becomes an Alpha:
At the end of the season, Jennifer attempts to kill Deucalion to get back at Kali for nearly killing her and get back at him for starting the whole pack killing, evil Alpha phrase. During the ending Jennifer surrounds herself in a circle of Mountain Ash. Again, Scott attempts to pass it and does pass it this time, destroying it whilst doing so. After doing he quotes the line "I'm an Alpha now" "Now get out of here before I kill you myself, I don't care what it does to the colour of my eyes". Deucalion then says "It won't change mine" before slicing her throat open. She eventually makes it back to the Nemeton, however, Peter is waiting there on her and ends up killing her taking her power. This is how he becomes so strong as he receives all of the strength and power she had received through her sacrifices. Not much people actually realise how that is how he got so strong is the later seasons.
How and Why he is so weak compared to the other Alphas:
A lot of people wonder why he is so weak and end up asking the question- Why is he so weak? He's a True Alpha? But they don't realise that he become an Alpha without killing someone, instead by pure will. When other Betas kill Alphas and become the Alpha themselves they are not just taking the power of becoming an Alpha but the strength that the werewolf had. When Derek killed Peter and became an alpha, he also took Peter's strength of which he had received when killing other people.
When Scott became an Alpha, all he received was the power of an Alpha i.e. the red eyes and the ability to turn people into werewolves/betas. He didn't receive any more strength than he already had gained with becoming a Beta. This is why he was so weak compared to other Alphas as they had received more strength than they already had, whenever Scott hadn't. You might be thinking that whenever Scott became the True Alpha he did become much stronger but that wasn't because he became an Alpha, that was because his pack was around him and as we already know, Alphas get stronger in a pack. When fighting alone he is weak but when fighting in a pack, he is much stronger.
A simple and short answer to his question would be that whenever Betas kill Alphas they also take that Alphas strength along with the Alpha abilities. Scott became an Alpha through pure will, without killing someone, this means he received no extra strength, but instead the ability to cross mountain ash.