Well, because of the alliteration.
And because Mondays have so much else working against them. They signal the end of the weekend, the beginning of the work week (though in the current economy, I suppose that's something to be grateful for...digressing...), the need to rise before the sun wakes up, the need to run the errands that couldn't be done over the weekend.
Mondays set the tone for the rest of the week. And, if you're like me (and Garfield), it tends to be the day on which anything that can go pear shaped, does go pear shaped. So, it needs all the help it can get.
(But mostly....it's because of the alliteration.)
“Music is life, and life is not a business.”- Iggy Pop