Why does anybody quit anything? Usually it is a combination of factors.
They're disillusioned:
Often people fall in love with the idea of something without having an understanding of the reality of it. Most people I know wish they could play the piano as well as a concert pianist (myself included). A lot of people I know are intrigued with the idea of climbing mountains, trekking through Nepal, travelling around the world on a motorcycle or bike, yadda, yadda, yadda. The reality of any activity is often very different from what the outside word sees in the news clips.
They are discouraged:
I remember a story about 4 frogs who fell into a deep, muddy hole. There was a great roar of horror as, up above, in safety, the rest of the frog tribe looked down at them. All four frogs jumped with all their might. As exhaustion began to take its toll the frogs up above starting yelling down, "There's no hope! Give up! You have no chance! There's nothing we can do!" 3 of the frogs simply gave up and were overcome by the mud. The fourth frog however didn't give up. He jumped even higher. The crowd of onlookers yelled even louder, "It will never work! You're doomed! GIVE UP!" The fourth frog didn't stop. With his last bit of effort he jumped the jump of a lifetime, sprung free of the mud, cleared the side of the hole, and landed in an exhausted heap amongst the throng of onlookers. The other frogs were amazed. They all gathered around to ask how he had done it and why he hadn't given up like they had told him to. It was then that they found out the fourth frog was deaf. He said, "Everytime I thought of giving up I looked up and saw everyone up above yelling at me. I thought you were all telling not to give up."
Having an environment of encouragement can go a long way towards keeping people involved in something. Not having that environment has the opposite effect, very quickly.
They have other priorities:
Let's face it, life sometimes throws us curve balls and we move towards other things that seem to feel more in line with where we want to go or who we want or think we need to be.
They lack a personal connection:
I don't think you can truly quit something that is a part of you, as m1469 suggests by coming back to the piano. Many people never develop a personal connection to music. It is something that comes off the radio and it fills up silence. I know many people who can listen to music but never really hear it. It doesn't affect them on any real emotional level. Without an emotional connection to something it is easy to give up. This area gets back to the question that has been asked several times on Piano Forum, "Why do you play the piano?"
Don't know if I added anything new to the discussion, but that's my $.03.
Jef