It already is and has been doing since 1997. The 'let' bit was negotiated by the British in the run up to 1997, so it is not a question of not doing or of doing what it is told. The Basic Law, for that is what the result of those negotiations is called, is to last 50 years. What will happen then is anyone's guess, just as it is in America.
We have the rule of law, free speech. We drive on the left whereas China drives on the right. Our average salaries are many times those of china and we have a partly elected legislative council which does very little one way or the other. Our Chief Executive is agreed with china and is, to all intents and purposes, a puppet, but the various burgeoning democracy parties bear little allegiance. The last time China tried to throw its weight around was over Article 23 (look it up) and hundreds of thousands took to the streets and prevented it.
China, however, is not the big, bad bogeyman your propaganda would have you believe. Its purpose is to bring China into the 21st century and, as you must realise, a growth and development programme like that cannot function under what you call democracy. What has taken you several hundred years China is achieving in a few tens. It cannot be judged according to western principles and certainly cannot be judged by Americans who are fed as much anti chinese propaganda as the Chinese are fed pro chinese propaganda. To your country China has taken the place of the USSR and Cuba. A totally fictitious enemy. But you like being scared shitless so I'll leave you to it.