Mendel developed pure lines of peas for his experiments because he wanted them to be

The answer is dominant homozygotes.

At the beginning of his work, the term dominant homozygote did not exist. However, this is what Mendel had accomplished when making his pure breed peas.
When developing the pure breeds for a certain trait (shape, color etc) Mendel wanted to design plants which will only pass their dominant trait to their offspring. By making these pure breeds and following the patterns of the inheritance he was able to discover the fundamental principles of genetics.