Why Orwell did not like Esperanto?

Polish linguist and pacifist L.L. Zamenhof invented Esperanto ('hope '), which he believed could be an international lingua franca or second language.
Because it had no irregular verbs and phonetic spelling, Esperanto was a 'perfect' language. It also had no associations with a particular nationality or country.

Orwell

George Orwell's aunt was an early proponent of Esperanto. When he stayed with her in Paris, she insisted on speaking this new language. 

The political element of Esperanto particularly disturbed Orwell. He believed that an attempt to control and direct language was a defining feature of totalitarianism.

Newspeak, the language Orwell created for his dystopian novel 1984, is clearly a variant of Esperanto.

More on Esperanto here