Another reason, and more important, was because most Indian princes were “Maharajas”, or “Grand Kings”, and thanks to title inflation that was now the standard Indian title. Rulers suzerain to the British monarch had titles superior to him/her, which made the government of Britain uncomfortable. There’s also the fact that Britain won the title through conquest of the Mughal Empire.
Another thing, if Bismarck did unite the Holy Roman Empire and put the Prussian King as Holy Roman Emperor, and created the Holy German Empire, would Bismarck become Imperial Vicar?