Why was it important for mendel to study such a large sample of pea plants to determine the probability of inheritance?

The answer is because the results will be more accurate if there is a larger sample size. 

No matter what research is, the larger sample size is, the more accurately will be the results and vice versa. The larger samples increase a chance of significance because they reflect the population mean more reliably.
With small sample, the chance for false conclusions is higher.

h i mean ok but yeah gotta tell me