Why people preserve dead bodies?

I don't know what the actual reason for doing this is, but I also know that it is not a new practice. Like you've mentioned, the kings and pharohs of Egypt had their bodies preserved and there have even been some Catholic Saints that have been preserved in this way. My guess would be that the reason that Michael Jackson asked that his body be preserved would be so that people in later generations would be able to admire him.

MJ has made a name for himself that i think the younger generation deserve to see his preserved remains. Unlike most of the famous people where all you see are just their clothes and movies. Preserving MJ's body would be as close as you can get to him being alive.

Hopes of being resurrected or something...

Well he thought even his body should live for ever, ignorant at that, cause after all the King of Pop, will live for ever anyway, because of his music and art.

I did not hear this but do not doubt it is true. So why not go cryonics to preserve himself and possibly be brought back? It was not in the will or it probably would have been done, lol. I guess you can also do the phermeldahide way. The reason for this is to be able to mummify the person and he would have a body for the after life or in the afterlife .

I think Michael Jackson and those kings and queens wanted to preserve their body because they loved their bodies very much. Those kings and queens used various beauty regimens even a long time ago. Michael Jackson did many surgeries on his face and I presume even on his body to achieve his desired looks.

If a genius brain like einsteins be preserved why not that by king of pop? :D

Because it's part of some countries' religious belief.

Egypt, the mummies were preserved because of their belief in the afterlife. That is why they buried their riches along with them. Another reason is for medical purposes... cadavers :)