Firstly, after the passage of the Royal Titles Act of 1876, Queen Victoria and her successors held the title of Emperor or Empress of India while remaining King or Queen of their other realms. As for why this was the case only for India, you may be interested in the answer /u/WhenTheRvlutionComes gave here https://www.reddit.com/r/AskHistorians/comments/1qydwx/why_were_british_monarchs_not_referred_to_as/ or /u/jschooltiger here https://www.reddit.com/r/AskHistorians/comments/3ptbu7/why_isnt_the_queen_of_england_titled_empress/