Why would a pea plant that is heterozygous for the tallness trait have the dominant phenotype?

Dominant character is expressed in both the condition i.e. heterozygous and homozygous.

Explanation:

In genetic the dominant allele is signified by capital letter and the allele which is recessive is signified by the smaller letter. The dominant character can be expressed in both the condition that is homozygous dominant(TT) or heterozygous dominant(Tt).  

Therefore even the presence of a single allele of dominant character in the gene, the dominant character can mask or suppress the recessive trait from being appeared in the phenotype. Therefore pea plant that is heterozygous for the tallness trait have the dominant phenotype.