Easy.... Trump was not allowed to block people since it was in effect the Government preventing free speech.

You can't speak to Trump right now on Twitter because Twitter has blocked his account.

You would no doubt be aware that the First Amendment (freedom of speech etc) only applies to acts of the Congress/Government. A non-governmental entity such as a company can block whatever speech they like. TDW for example, even blocks people who have not violated their own rules.

The Constitution really is an interesting document. You should read it some time.