According to AYATOLLAH KHAMENEI LETTER TO THE YOUTH OF THE WEST, Why does the power in the world want Islamic thought to be marginalized?

Because even Muslim leaders know that certain members of THEIR faith have picked a fight with the Western world they simply cannot win.

The powerful in Muslim nations.... would like to retain that power. The wealthy in Muslim nations would like to maintain that wealth.

Adults (specifically people that have witnessed every terrorist attack going back the last 20 years or so) don't see a whole lot of "change" in the Muslim world and the change they have seen like the "Arab Spring" (they generally hoped that would make the Middle East more democratic)... have seen things only get worse. The attacks simply haven't stopped. The wars have gotten worse. The extremism has gotten worse. The plight of women has gotten worse.

Any adults that are paying attention.... their mind is not so easy to change at this point. So... the Ayatollah thinks that he can speak to the generations that have not seen what the Muslim world has become..... and perhaps change the minds of some.

Personally I think he's wrong.... and VASTLY overestimates the power of an Iranian Ayatollah anywhere outside of Iran.... but he can try if he likes. He just shouldn't be surprised when all he hears in response is laughter. The young like freedom. The young like to say as they please. And Islam attempts to crush both.