Districts are drawn more or less arbitrarily, and so their residents don't necessarily represent a single coherent political bloc. In practice, this means that you can have a party getting 30-40% of the vote in literally every district, and end up with zero representation.

Worse yet, because most FPTP elections only require plurality for victory, you can have a party get less than 50% of the vote in each district (but more than any other party), and still take all the seats. In which case you literally have the minority running the country - so much for "people have spoken".