Notice that not ONE other Lion in this movie was a good fighter. Simba was obviously weak since he had no proper training and lived a carefree life with Timon and Pumbaa.
And Scar isn't that good of a fighter, hense why he used the hyenas and his security blanket.
I've noticed in both movies, the first and second one, that Simba isn't particularly a good fighter. Nala has beaten him multiple times in the first movie and Scar had most of the upper hand until Simba used a trick that Nala taught him.
It's even more noticeable in Simba's Pride. I don't think any male lion could prevail against that many lionesses piling on him at once, but at one point all Zira had to do was strike Simba with one paw and he was down! I mean...come on, Simba.