Why are peas in mendelian genetics different?
You are confounding the concept of dominance with the one of high frequency. The dominant phenotype might be at lower frequency than the recessive phenotype.

Dominance


  Dominance in genetics is a relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus


Trait frequency

Let's imagine a case where A is dominance and a recessive. Let, $f$ be the frequency of A alleles in the population. The frequency of individuals that have the dominant phenotype is the frequency of AA individuals plus the frequency of Aa individuals. Under Hard-Weinberg assumptions, the frequency of individuals having the dominant phenotype is therefore

$$f^2 + 2 f (1-f)$$

The frequency of individuals having the recessive phenotype is $(1-f)^2$.

If 

$$f^2 + 2 f (1-f) &lt; (1-f)^2$$

reorganizing the in-equation yield to

$$f &lt; 1 - \frac{1}{\sqrt{2}}$$

So under the condition that $f &lt; 1 - \frac{1}{\sqrt{2}}$, the dominant phenotype is present at lower frequency than the recessive phenotype.

If the calculations were unclear to you, then you should have a look at the wikipedia article of Hardy-Weinberg principle.