Because the primary fixation of Co2 is mediated via phosphoenol pyeuvate (PEP) carboxylase.
The fixation of Co2 in C4 plants takes place in two places and by different organic compounds. Phosphoenol pyeuvate (PEP) is found in mesophyll cells which primarily fixes atmospheric Co2 into oxaloacetic (4c) . RUBISCO is present in bundle sheath cells when final fixation if Co2 in hero's sugar takes place. Co2 is primarily fixed by phosphoenol pyeuvate (PEP) Caboxylase because this enzyme has greater affinity to Co2 then RUBISCO.

Introduction:
C4 plants are a group of plants that have evolved a specialized mechanism to enhance photosynthesis and increase their efficiency in environments with high temperatures and low water availability. Unlike C3 plants, which constitute the majority of plant species, C4 plants have an additional step in their photosynthetic pathway, known as the C4 pathway. This mechanism allows them to overcome the limitations imposed by high temperatures and low CO2 concentrations.

Less Limitation by Atmospheric CO2 Levels:
C4 plants exhibit a lower limitation by atmospheric CO2 levels compared to C3 plants due to several reasons:

1. Carbon Concentrating Mechanism:
C4 plants possess a carbon concentrating mechanism (CCM) that helps them to overcome the inefficiency caused by low CO2 levels. In this mechanism, carbon dioxide is initially fixed into a 4-carbon compound in the mesophyll cells and then transported to bundle-sheath cells where it is decarboxylated to release CO2 again. This way, the CO2 concentration in the bundle-sheath cells is much higher than in the surrounding atmosphere, ensuring efficient photosynthesis even when the atmospheric CO2 levels are low.

2. Increased CO2 Affinity of Enzymes:
The enzymes involved in the C4 pathway have a higher affinity for CO2 compared to the enzymes in C3 plants. This enables C4 plants to effectively capture and fix even trace amounts of CO2 from the atmosphere for photosynthesis. Consequently, they can maintain a high rate of photosynthesis even when atmospheric CO2 levels are low.

3. Reduced Photorespiration:
C4 plants have a reduced tendency for photorespiration, a process that occurs in C3 plants when the enzyme rubisco mistakenly binds with oxygen instead of CO2. Photorespiration consumes energy and reduces the efficiency of photosynthesis. The C4 pathway minimizes photorespiration by concentrating CO2 in the bundle-sheath cells, where rubisco is shielded from oxygen, resulting in a higher photosynthetic efficiency.

4. Favorable Stomatal Behavior:
C4 plants exhibit a different stomatal behavior compared to C3 plants. Stomata are the tiny openings on leaves that regulate gas exchange. C4 plants tend to keep their stomata partially closed during the hottest part of the day, reducing water loss through transpiration. This adaptation helps to conserve water and maintain higher internal CO2 concentrations even when atmospheric CO2 levels are low.

Conclusion:
In conclusion, C4 plants have evolved a carbon concentrating mechanism, possess enzymes with higher CO2 affinity, minimize photorespiration, and exhibit favorable stomatal behavior. These adaptations collectively allow them to be less limited by atmospheric CO2 levels compared to C3 plants. This advantage enables C4 plants to thrive in high-temperature environments with low CO2 availability, making them successful in arid regions and allowing them to outcompete C3 plants in such conditions.