Why does Khamenei have a Twitter account and the president of the Free World does not?

Well, now you know who Twitter sides with.

Trump is a bigger threat to U.S.  

Who is this "president of the Free World" about whom you so ignorantly babble?  I didn't know there was ever such a person.  Tell us about them.

As to why one person has an account somewhere and someone else doesn't, there can be a thousand good, intelligent reasons.  Based on your question, I doubt you'd understand any of them.

Because even Khamenei didn't use it to call on Americans to do what Trump told them to do.

Because Khamenei follows the Twitter's rules for what he can post there.

Khamenei is subject to the same terms and conditions as other Twitter users. He recently had his account suspended until he removed a tweet suggesting coronavirus vaccines made in the United States and Britain are “untrustworthy”, which violated Twitter's COVID-19 misleading information policy. So he has at least one strike towards Twitter's "three strikes and you're banned" policy that Trump fell foul of. If anything, Twitter had been too lenient with Trump for too long.

Because he violated all the rules for years and they let him get away with it, but then he tried to overthrow the government and they had to stop him.