I agree, Putin, that the divide-and-rule strategy worked extremely well over, playing on the Hindu-Muslim tensions, etc. Related to this would be the fact that Britain, out of all the nations left in the vacuum of the crumbling Mughal empire, was the strongest. Their money and military strength allowed them to take over smaller kingdoms, principalities, etc, who then supplied them with soldiers, increasing their manpower. On sea, most, if not all, Indian nations were heavily out-classed by the Royal Navy (indeed by all European navies). On land, they heavily outnumbered the British. Even their artillery was supposed to be better than what Wellesley had (I can't remember the quote, but he did praise their artillery). However their tactics completely failed them. Many soldiers still used bows and arrows, cannons were given very little importance, while strong emphasis was placed on their light cavalry. Most infantry hardly used guns, preferring to charge with sabers and pikes. Look at Assaye and Plassey. There was around 40 years in between them, but tactics hadn't changed much by the time Wellesley appeared.
I'm sure there are other reasons, but that's all I can think of at the moment.