Hong Kong is at the PRC's doorstep, has little arable land and few natural resources, the economy base around transportation, trade, and manufacturing.
As someone living in Hong Kong, I will like to add to this by pointing out that what little arable land Hong Kong had were almost entirely in the New Territories, and were no where near enough to feed even a small portion of the population. Back in WW2 during the Japanese occupation from 1941-45, the IJA military administration understood this, which was why one of the first things they did was to deport a large portion of the city's population up north into the Mainland, even allowing them to flee into KMT or communist control territories, since that was the only way to prevent starvation from breaking out... And this was during the 1940s! When the city still had much more farmland, and a much lower population.
The same goes for the supply of fresh water, even with all the reservoirs in the city (The largest ones all being in the New Territories, BTW), 70% of the city's fresh water supplies came from the Dongjiang River, which is under PRC control (And in OTL, even during the height of the Cultural Revolution, the taps were never turned off). The British did experimented with seawater desalination back in the 1970s', but that was quickly abandoned due to it costing way too much.
I hate to say this, but the Mainland government held all the card, and OTL was about the best deal that we could have gotten.