I think there are some premises in the question that should be re-examined.

Regarding the Sepoy Rebellion, I'm not sure the toll was "the deaths of millions". I have absolutely no doubt that the figures are imprecise, but I'm seeing estimates in the range of 800,000, including those who died of malnutrition and disease from the knock-on effects of the 1857-1858 war. I would defer to an Indian history expert here though.

Regarding the American Revolution death toll, records are fragmentary and incomplete, so we need to do a bit of work assembling this figures. Official dead tend to be for Americans who died in battle, so around 6,800, but to this should be added an estimated 17,000 who died of disease, especially as prisoners of war. Possibly something like 24,000 to 25,000 British service personnel died in the American revolution, from battle or disease, or went missing. Something like 2,112 French died in the war. There were substantial native casualties, especially among the Iroquois, although frankly I can't find anyone even trying to estimate those.

On top of this, if we're really going to compare apples to apples between the American Revolution and the Sepoy Rebellion, than we need to add in the deaths suffered by Americans in a major smallpox epidemic that lasted from 1775 to 1782 and was greatly abetted by the dislocations and privations of the war. Elizabeth Fenn in Pox Americana: The Great Smallpox Epidemic of 1775-82 says the conservative death count from this epidemic is in the range of 125,000. While a great number of these deaths are already counted in the military deaths from disease above, it nevertheless had a major impact on the civilian population as well.

So to put together these incomplete figures, we're maybe talking about 150,000-175,000 deaths connected to the American Revolution in North America. To put things further into perspective, it's worth remembering that the 13 Colonies had an estimated 1770 population of 2.5 million, while India had a population of 239 million according to the 1871 Census, or something like 100 times the American population. And furthermore, this is in an eight-year long war (1775-1783) that ultimately involved Britain being at war with France, Spain and the Netherlands, and fighting in the West Indies, India proper, and Europe, in addition to North America, while the 1857-1858 war was restricted mostly to northern India (so no foreign power involvement, and even a great deal of Indian states and British possessions in India either remaining neutral or openly supporting the British).

Edited to include a few points

Having read lots of books on this period in British history - of many sorts of angles / coming from many different perspectives - there is a historical consensus (ie. it is not controversial to say) that not only the British war efforts, but much of their entire colonial empire throughout the world depended on resources from India. It was always thought that if they were to lose India, they would become incredibly weakened, and the rest of the empire could collapse itself.

Thus, it was a far better decision to retain power over the South Asian region (which, mind you, was even bigger than India is today, including what's now modern Pakistan, Bangladesh, Burma, etc.) rather than fight for North American colonies which were providing basically no resources in comparison. Just think: India was heavily populated, resource rich, and much of the population was under subjugation and able to be controlled in terms of what they farmed, what resources the collected, how labor is allocated, plus huge taxes could be levied. Whereas in the U.S., barely any output is coming out in comparison, people are willing to wage war so as not to pay taxes to a government they no longer identify with, and even trade with the U.S. could be relatively expensive whereas resource collection from India only required the cost of military subjugation and administration. That's not to say that the wars against Indian empires were inexpensive, though, or against other colonial powers in the region (France, Portugal, the race to take things from what was feared Russia would go for in the Afghani regions). It's just to say that it was more worthwhile because of how severely enriched the English were becoming; the reward was far bigger than trying to control an early America.

A side note: one such costly/difficult war was between the British and Tipu Sultan, and following his defeat, it was his rockets/missiles that they subsequently used against the Americans in war. It was Tipu's "rockets' red glare" that the Americans saw and is referred to in the Star Spangled Banner. Moreover, the Star Spangled Banner was composed by someone who saw these rockets while boarded on the HMS Minden - a warship that was built, you guessed it, in Bombay, India!

In a sense, that's kind of what happened in the 20th century (other British colonies became independent following the liberation/partition of India), but that's also a totally different period in time, and the British were weakened by World War 2 in a way that made managing India (which was already being grossly mismanaged, suffering from incredible poverty, and having huge violent and non-violent but non-cooperative freedom movements) was simply too burdensome.