The simple answer is that they **did**.

The 1876 Royal Titles Act authorized Queen Victoria to assume the title of "Empress of India".

From that time until the partition of India and the independence of Pakistan in 1947, her successors used the title of Emperor of India.

The British Monarch was the King-Emperor all over his/her Dominions and in India itself, the Emperor of India. ironically the only time the title of "Indian Emperor" was ever used in India itself was for a British monarch, previous Indian Emperors all the way back to Chandragupta Maurya in 320 BC had used various titles, some of which translate very close to but not exactly to Emperor of India, the Mughals had used Shainshah-Hind a Farsi term which means something like "King of the Kings of India" and is usually translated as Emperor of India. The translation of the term "Emperor of India" into Urdu and Hindi was "Kaiser-Hind" literally "Ceaser of India"., which was a weird translation since the word "Kaiser" was used almost exclusively in relation to the Romans and Byzantines, it was also a fairly common personal name,

As to why it was limited to "India" and not generally, well thats been a major question. Officially and in actuality, it was since Government advice was that the title "Emperess" should be limited to India and such advice is by convention binding on the monarch. There have been some claims that Queen Victoria wanted to be an "Emperess" of all, without distinction, but they have been difficult to verify here, though if someone else has sources I would be delighted. The idea that the British monarch should be delcared Emperor of India was one with a lot of support from the late 1830's onwards as the British became the power in India, previously they had been a power.

The title of Emperor was also suggested for George III when the Union with Ireland was effected in 1800, but not adopted.

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Sources

L. A. Knight, "The Royal Titles Act and India", The Historical Journal, Cambridge University Press, Vol. 11, No. 3 (1968), pp. 488–489.

B.S. Cohn, "Representing Authority in Victorian India", in E. Hobsbawm and T. Ranger (eds.), The Invention of Tradition