Is it not possible to get a heart transplant?
It is possible
What factor would diminish the chance of success of a lung transplant but is not a factor in a heart transplant?
If you are a healthy person and your heart is in good condition then nothing will happen if you don't get a heart transplant. You don't need one. If you are suffering from some sort of heart failure and there are no alternative treatments than a heart transplant than you will eventually die when your heart stops doing its job.
You go and see a cardiologist at a hospital with a heart-transplant unit. If they think that you need a heart-transplant, you will be put through a "transplant assessment" to see if you are suitable. (Blood tests, MRI's, ultrasounds, EEG's, ECG's, psychological evaluations etc...) If you are suitable, you will be added to the waiting list.