Some (hot-climate origin, usually) religions forbid it, in religious scripture. Scholars also speculate that the prohibition may have come about in some regions because they had no form of cold storage. In hot climates, milk will go bad in a few hours, unlike dried/cured meat, and if cooked together such a dish can make people ill. Other than that, there's no dietary, nutritional, or health reason to avoid eating milk and meat together.
I've heard that both milk and meat require some of the same digestive enzymes to be broken down and that if you eat them at the same time your body can't process them as efficiently as if you had them separately. If this is in fact true then it wouldn't so much be bad for you, but you wouldn't get the full nutritional benefit.