Why does the rain in Spain fall mainly in the plains?

As you certainly know, it is a song from My Fair Lady, to teach a cockney girl to pronounce correctly the sound "-ain." I remember once being in an English pub in Benidorm, Spain, and hearing the British owner saying: "Do you want your fish cake on a plate, mate?" I knew then that he was from north-east England! ;-)

Now, on a meteorology point of view, the rain in Spain falls mainly in the north, as low pressures move along the Pyrenees, and in the south, as they enter the Strait of Gibraltar. This is why, for example, Andalucia is greener than the Alicante province.

Because most of the people live in plains and they are ignorant of the rain that falls on mountains.

Physics.

Mediterranean climate patterns

it is controlled by the government.

...because it rhymes.