There are several reasons.
The spitfire has bigger ADS moving penalty(aim down sight) -60%, in comparison to flatline, which has -50%. Slight difference, but still.
It may have the bigger magazine, but it fires slower than flatline. What means, that fast firing weapons can destroy you at close. More likely, than in case of having the flatline.
Flatline can equip anvil receiver, what allows you to deal a lot of damage by one shot. Spitfire cannot do that.
To be honest, i was never able to perform well with spitfire, from machine guns i prefer L-STAR. But i won't give up on it.
Love the sound, when you shoot, and your magazine gradually becomes emptier. When you fire last bullet, there is a cool sound too.👍