Answer:
Because of the polination characteristics of the pea plant.
Step-by-step explanation:
Mendel had various reasons to use pea plants for his genetic experiments such as the observability of the pea's traits, ease of reproduction etc. but the main reason he used them is because they allowed him to have controlled mating which made the traits more controllable and therefore Mendel was sure of his observations and results. This was possible because the pea plants, during their flowering periods, are protected from alien pollinating, therefore he could control the polination process of the experiment, which was crucial in terms of accuracy and reliability of the results.
I hope this answer helps.