To make sure all his heredity experiments were completed.
Genetic experiments, especially those investigating heredity and phenotypes take years to complete. This is one of the primary reasons genetic experiments (especially heredity) have never been conducted on humans. Scientists, instead, resort to a model system.
A model system is an organism that has been designed to make it easy for scientists to conduct experiments and reach reasonable conclusions.
Mendel chose the pea plant as a subject for its ability to replicate rapidly and produce multiple generations of offspring in a short time. This enabled him to study each generation's phenotypes and genotypes in detail. He could easily give sound conclusions on how a traitor gene travels down different generations.