That's a simple question with a complex answer, as many things can affect pollination success. Here are a few:
- Maturity of the flower at time of pollination.
- Age of the pollinia.
- Relative size of the parents.
- Genetic compatibility of the parents.
- Health of the "mother" plant.
Sometimes it's simply a matter of statistics - commercial nurseries tend to have MANY plants on hand, so when hybridizing, the the probability of a cross "taking" is improved - you may try it once while they might try the same one 50 times.
Sometimes a new plant is just a variety of an existing one - an especially good "odd ball", if you will, so again, the size of their plant population increases the chance they'll see them.