Why did British fail to stop Japanese?

Answer 1

Britain failed to prevent the Japanese from invading Singapore because Japan had air superiority over the region.

Most of England's air power was in Europe fighting Hitler's armies and defending England from invasion. Therefore England greatly relied the United States to provide air cover.

So, in respect, your question is incorrect. The real question is: Why did the USA fail to support the UK with air support when Japan invaded Singapore?

Because of the Japanese airstrikes, British forces were concentrating on defending Singapore from a head-on attack by Japan from the borders, air and sea.

However, Japanese infantry managed to out-flank the British defenders by riding bicycles through the harsh Malaysian jungles.

Because it's near impossible to get tanks and armoured vehicles through the Malaysian jungles, there were no British tanks in the area the Japanese flanked from, because it wasn't believed that Japan could successfully invade from that position.

The British coastal guns protecting the Coastlines of Singapore were turned around to face the land and aimed at the invading Japanese infantry, however because these guns and ammunition were designed to be fired at ships, they were very clumsy at firing on land targets.

Had the US provided decent air coverage to protect the borders of Singapore, the British could have repelled the Japanese easily.

Answer 2

The British and Japanese were pitted against each other when Japan invaded Singapore. The reason why the British failed to stop the Japanese was that the Japanese outsmarted the British forces, and they also had superior air support.