Medium height (like in people) and other traits that seem like a mixture of two extremes are often a result of incomplete dominance. For example, a red and white flower are bred to produce an offspring with pink petals.
Mendelian genetics does not include incomplete dominance (which is classified as, surprisingly, non-Mendelian genetics).
Basically, Mendel got **very lucky** with his choice of plant. Pea plant height is strictly dominant, meaning one dominant allele results in tall plants, regardless of the identity of the second inherited allele. This is a consequence of the genetic makeup of pea plants. Had he tried a similar experiment with snapdragon flower color, he would be very confused.
(See < for snapdragon incomplete dominance example.)