That’s why. Basically every other player has more experience than you here, most with years of gameplay under their belts. You have a lot of practicing to do to catch up. You can’t expect to be topping the charts all the time if you just started.
I can understand that this can be frustrating and matchmaking is still incredibly flawed for most games, making it even tougher on newer players. Dirty Bomb is also not the most newbie friendly shooter either. It emphasizes skill more than most other shooters to be honest (which is why I love it so much).
The best advice I can give you is to try and enjoy yourself and not get so absorbed in improving. Being focused on that aspect will only lead to frustration. Getting better will come naturally as you play the game.