I'm guessing this is because the particle pairs have a certain mass, and this mass is related to the energy of the photon by e=mc^2. The energy is related to λ by, e = hc/λ
This leads us to hc/λ = mc^2
which gives, h/λ = mc
h and c are constants, so λ is inversely proportional to m. You can't create the particles, unless you have enough energy. There's a minimum energy requirement. However, if you have more than necessary energy, this "extra" energy is the kinetic energy of the particles.
All of this, of course, is just a guess.