Why might carbon-13 be more abundant than carbon-14 despite them both being
isotopes?

The reason carbon-13 is more abundant than carbon-14 despite them both being  isotopes is because carbon-13 is stable and doesn't decay into other elements, unlike carbon 14.

Explanation:

Carbon isotopes come in three forms. By far the most common isotope of carbon is carbon-12 (12C), which contains six neutrons in addition to its six protons. The next heaviest carbon isotope, carbon-13 (13C), has seven neutrons. Both 12C and 13C are called stable isotopes since they do not decay into other forms or elements over time. The rare carbon-14 (14C) isotope contains eight neutrons in its nucleus. Unlike 12C and 13C, this isotope is unstable, or radioactive. Over time, a 14C atom will decay into a stable product.