Since there is both a nitrogen and carbon cycle why is there more nitrogen in the atmosphere than carbon?
Because nitrogen is does not react chemically with a lot of other elements besides oxygen. Carbon is very reactive and can form a number of chemical compounds.
The nitrogen in the Earth's atmosphere was thought to have been derived from volcanic eruptions over 4 billion years ago. Nitrogen reacts with oxygen, but not a lot of of other elements except those in biological systems. So, this means that most of the nitrogen that was formed 4 billion years ago, is still hanging around.
In contrast, carbon reacts with oxygen to form CO and CO2 but also to form calcium carbonate ( shell of sea creatures) and a number of other biological reactions and crystal reactions. So, carbon tends to cycle throughout the atmosphere at various time scales. Most carbon is actually locked up in calcium carbonate rocks (limestones), and fossil fuels deep in the Earth and so is kind of trapped.