Because they had a good officer corps producing some ridiculous military geniuses in their age of empire. As an example, the Duke of Wellington (Arthur Wellesley) was a monster who was unstoppable in the military conquest of India. Many other British commanders failed battling Indian states, but he seemed to win just about every battle, at times being both outnumbered and outarmed.
I would go on to say that it wasn't for that man, it is likely the British conquest of India would have been confined to only a limited territory. Indian states were modernizing and militarizing rapidly (relatively for that era), so any delays in conquest would have made India a hard nut to crack.
