In a hysteresis curve, why the area of the curve proportional to thermal energy per unit volume developed in a material?
And why is it that the materials with larger area under hysteresis curve can't be easily magnetized while the ones with smaller area are easily magnetized?
Because more excitation that you need to provide to the material in order for it to move it's magnetic dipoles or Weiss domains, the more energy you have expended. The more you have to provide, the more a hysteresis curve widens. Plot it on a paper and you Will find this to be so.
Why some materials need more energy to turn their magnetic dipoles is because of their structure. It takes some energy to push the magnetic dipoles away from their current lowest energy point. They are often stuck in certain positions because of impurities preventing the dipole domains to rotate.
And this explains your second part of question. The wider area means more energy is needed to change their magnetization. This means that thermal effects take longer to randomize those magnetic dipole orientations inside a material. It is for the same reason required to spend more energy to turn them.
