My theory behind this was basically because of balancing issues. Take MAG for example.
The level cap was at 70, and you were fairly beast when you reached this level, because now you’ve unlocked pretty much everything you wanted to. However, if a level 1 and a level 70 met in a firefight, and they both were similar in talent, the level 70 would cream the level 1.
So essentially my theory is that the lower level cap gives incentive and customization features without giving higher leveled players a ridiculous advantage.
Then again I haven’t played Brink yet, and this is just my theory based on what I’ve heard.