Lots of reasons.
They are very intuitive and easy to play. Piano = touch key, note plays (as opposed to learning how to blow into a saxophone properly)
They can be played/ practiced quietly. (Compare to practicing a trumpet)
They can be accompanied by singing (compare to drums or an instrument that requires your mouth to play)
They play chords, giving much greater musical depth.
String instruments are easy to troubleshoot/repair or even build yourself (guitar, not piano so much).
They are very easily digitized. Getting an oboe onto a quality recording requires a lot of experience.
Lastly, they are already so ubiquitous that their popularity feeds itself. If you like Coldplay you can learn their songs on piano, play along, and play for/with people who also know them.
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