Why did Buddhism nearly become extinct in Sri Lanka?

I read when the British came to Sri Lanka (Ceylon) during the colonial era, Buddhism was virtually extinct in Sri Lanka and has degenerated into very low forms of superstition. However, because some Sri Lankans wanted to stop British Christianity, they requested the Burmese Sangha to reinstate a Sri Lankan monastic Sangha. I read some intellectual Sri Lankan laymen had debates with Christians, where both sides (logically) accused the other side of superstition. Why did Buddhism become virtually extinct in Sri Lanka? Was that 'extinction' somehow related to the form of Buddhism or how it was practised, or something else?