The piano provides a certain amount of early gratification because students are capable of playing melodies early on, and get enjoyment by playing recognizable favorites such as Mary Had a Little Lamb, Old MacDonald and other tunes.
The piano student must play the correct notes in the correct rhythm, and if that is achieve the melody is recognizable. On other instruments, meeting these two simple goals is not any where near as easy. Playing out of tune and producing sounds that are unpleasant are too much an issue for the beginning instrument student.
First attempts at singing can be wrought with these same problems, and this does not indicate a lack of talent. Many students who cannot carry a tune when they first begin piano lessons will be able to carry a tune after 2 or 3 years, simply because of learning the musical concepts and hearing the melodies played on the piano.