Because they were pretty much obliged to after Thatcher's infamous fall in Beijing. The signatures were already on the paper, and revoking it would have been seen as the UK giving the finger to China, which would likely have then led to war. Also, there might have been the lingering and sobering reality of how easily Goa had fallen to India in 1961.
It wasn't a matter of they "wouldn't", it was more because they "couldn't". Deng won the entire proceedings of the Sino-British Joint Declaration, and Thatcher couldn't back the colony up - without the agreement, China would have strangled Hong Kong until she gave in. The British position was much worse than many people think.
Of course, nobody asked the good people of Hong Kong. But were the people given a say, it would have been a resounding "no" to reintegration with China - protests against Beijing's tyranny following Tiananmen were some of the most heated and furious in the world, and each June 4 we still hold candlelight vigils in memory of the people who died.