Asked and answered before from here

/u/WhenTheRvlutionComes via /u/RiceandBeansandChees

king/queen was just the title they traditionally held in their position as monarch of Great Britain. ... it's a different matter to just convert a pre-existing Queen title to an Empress one.

For one thing, it would inevitably raise suspicions of an attempt to instate absolute monarchy, which is antithetical to British tradition. It was, to some degree, fine to rule as Empress in a faraway colony, it wouldn't necessarily be acceptable to try to be Empress of Britain. Claiming the title in India also had more legitimacy than it would have in Britain, as the previous Mughals had ruled as more less "emperors", and Victoria was ruling over rajas, which are effectively equivalent to Kings, and an Emperor is, in one sense, a King of King. As well, India had a vast territorial extent and diversity compared to Britain.

Also ...

She couldn't be Empress of the whole British empire without parliaments approval