From what I remember of my rusty high school chemistry magnetism requires a certain relatively stable configuration of the valence electrons. I'm guessing that as temperature increases it's impossible for the atoms (or even just the electrons) to maintain that configuration because of the increased energy. The electrons might even get excited enough to jump to entirely different orbitals, but I'm not sure if it works that way.
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From what I remember of my rusty high school chemistry magnetism requires a certain relatively stable configuration of the valence electrons. I'm guessing that as temperature increases it's impossible for the atoms (or even just the electrons) to maintain that configuration because of the increased energy. The electrons might even get excited enough to jump to entirely different orbitals, but I'm not sure if it works that way.
In general, magnetism requires the material to be ordered, and heating an object up involves adding lots of random motion to the system.
That's what causes hot objects to glow, basically -- the electrons get pushed to higher orbitals, and then fall back down again, emitting light.